Experiments with Alternate Currents of High Potential and High Frequency (1892) — part 2 of 8

carefully soldered all around. It might be advisable, in a strictly scientific investigation, when accuracy is of great importance, to do away with the metal cover, as it might introduce many errors, principally on account of its com¬ plex action upon the coil, as a condenser of very small ca¬ pacity and as an electrostatic and electromagnetic screen. When the coil is used for such experiments as are here contemplated, the employment of the metkl cover offers some practical advantages, but these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the space between should, of course, not be too small, certainly not less than, say, five centimetres, but much more if possible; especially the two sides of the zinc box, which are at right angles to the axis of the coil, should be sufficiently remote from the latter, as otherwise they might impair its action and be a source of loss.

The coil consists of two spools of hard rubber R R, held apart at a distance of 10 centimetres by bolts c and nuts n, likewise of hard rubber. Each spool comprises a tube T of approximately 8 centimetres inside diameter, and 3 millimetres thick, upon which are screwed two flanges FF, 24 centimetres square, the space between the flanges being about 3 centimetres. The secondary, S S, of the best gutta percha-covered wire, has 26 layers, 10 turns in each, giving for each half a total of 260 turns. The two halves are wound oppositely and connected in series, the connec¬ tion between both being made over the primary. This disposition, besides being convenient, has the advantage that when the coil is well balanced — that is, when both of

17

its terminals Tx T1 are connected to bodies or devices of equal capacity — there is not much danger of breaking through to the primary, and the insulation between the primary and the secondary need not be thick. In using the coil it is advisable to attach to both terminals devices of nearly equal capacity, as, when the capacity of the termi¬ nals is not equal, sparks will be apt to pass to the primary. To avoid this, the middle point of the secondary may be connected to the primary, but this is not always practi¬ cable.

• The primary P P is wound in two parts, and oppositely, upon a wooden spool W, and the four ends are led out of the oil through hard rubber tubes t t. The ends of the secondary Tx Tx are also led out of the oil through rubber tubes tx tx of great thickness. The primary and second¬ ary layers are insulated by cotton cloth, the thickness of the insulation, of course, bearing some proportion to the difference of potential between the turns of the different layers. Each half of the primary has four layers, 24 turns in each, this giving a total of 96 turns. When both the parts are connected in series, this gives a ratio of conver¬ sion of about 1:2.7, and with the primaries in multiple, 1 : 5.4; but in operating with very rapidly alternating cur¬ rents this ratio does not convey even an approximate idea of the ratio of the E. M. Fs. in the primary and secondary circuits. The coil is held in position in the oil on wooden supports, there being about 5 centimetres thickness of oil all round. Where the oil is not specially needed, the space is filled with pieces of wood, and for this purpose princi¬ pally the wooden box B surrounding the whole is used.

18

The construction here shown is, of course, not the best on general principles, but I believe it is a good and convenient one for the production of effects in which an excessive potential and a very small current are needed.

In connection with the coil I use either the ordinary form of discharger or a modified form. In the former I have introduced two changes which secure some advantages, and which are obvious. If they are mentioned, it is only in the hope that some experimenter may find them of use

Fig. 4.— Arrangement of Improved Discharger and

Magnet.

One of the changes is that the adjustable knobs A and B (Fig. 4), of the discharger are held in jaws of brass, J J, bjr spring pressure, this allowing of turning them succes¬ sively into different positions, and so doing away with the tedious process of frequent polishing up.

The other change consists in the employment of a strong electromagnet N S, which is placed with its axis at right angles to the line joining the knobs A and B, and produces a strong magnetic field between them. The pole pieces of

10

the magnet are movable and properly formed so as to protrude between the brass knobs, in order to make the field as intense as possible; but to prevent the discharge from jumping to the magnet the pole pieces are protected by a layer of mica, M M, of sufficient thickness. x and s.> s., are screws for fastening the wires. On each side one of the screws is for large and the other for small wires. L L are screws for fixing in position the rods R R, which sup¬ port the knobs.

In another arrangement with the magnet I take the dis¬ charge between the rounded pole pieces themselves, which in such case are insulated and preferably provided with polished brass caps.

The employment of an intense magnetic field is of ad¬ vantage principally when the induction coil or transformer which charges the condenser is operated by currents of very low frequency. In such a case the number of the fundamental discharges between the knobs may be so small as to render the currents produced in the secondary unsuit¬ able for many experiments. The intense magnetic field then serves to blow out the arc between the knobs as soon as it is formed, and the fundamental discharges occur in quicker succession.

Instead of the magnet, a draught or blast of air may be employed with some advantage. In this case the arc is preferably established between the knobs A B , in Fig. 2 (the knobs a b being generally joined, or entirely done away with), as in this disposition the arc is long and un¬ steady, and is easily affected by the draught.

When a magnet is employed to break the arc, it is better to

choose the connection indicated diagrammatically in Fig. 5, as in this case the currents forming the arc are much more powerful, and the magnetic field exercises a greater influ¬ ence. The use of the magnet permits, however, of the arc being replaced by a vacuum tube, but I have encoun-

Fig. 5.— Arrangement with Low-Frequency Alter¬ nator and Improved Discharger.

tered great difficulties in working with an exhausted tube.

The other form of discharger used in these and similar experiments is indicated in Figs. 6 and 7. It consists of a number of brass pieces c c (Fig. 6), each of which comprises

Fig. 6.— Discharger with Multiple Gaps.

a spherical middle portion m with an extension e below — which is merely used to fasten the piece in a lathe when polishing up the discharging surface — and a column above, which consists of a knurled flange f surmounted by a threaded stem 1 carrying a nut n, by means of which a

r

wire is fastened to the column. The flange / conveniently serves for holding the brass piece when fastening the wire, and also for turning it in any position when it becomes necessary to present a fresh disharging surface. Two stout strips of hard rubber R R, with planed grooves g g (Fig. 7) to fit the middle portion of the pieces c c, serve to clamp the latter and hold them firmly in position by means of two bolts C C (of which only one is shown) passing through the ends of the strips.

In the use of this kind of discharger I have found three principal advantages over the ordinary form. First, the dielectric strength of a given total width of air space is greater when a great many small air gaps are used instead of one, which permits of working with a smaller length of air gap, and that means smaller loss and less deterioration of the metal; secondly by reason of splitting the arc up into smaller arcs, the polished surfaces are made to last much longer; and, thirdly, the apparatus affords some

gauge in the experiments. I usually set the pieces by putting between them sheets of uniform thickness at a cer¬ tain very small distance which is known from the experi¬ ments of Sir William Thomson to require a certain electro¬ motive force to be bridged by the spark.

It should, of course, be remembered that the sparking distance is much diminished as the frequency is increased. By taking any number of spaces the experimenter has a rough idea of the electromotive force, and he finds it easier to repeat an experiment, as he has not the trouble of setting the knobs again and again. With this kind of discharger I have been able to maintain an oscillating motion without any spark being visible with the naked eye between the knobs, and they would not show a very appreciable rise in temperature. This form of discharge also lends itself to many arrangements of condensers and circuits which are often very convenient and time-saving. I have used it preferably in a disposition similar to that indicated in Fig. 2, when the currents forming the arc are small.

I may here mention that I have also used dischargers with single or multiple air gaps, in which the discharge surfaces were rotated with great speed. No particular advantage was, however, gained by this method, except in cases where the currents from the condenser were large and the keeping cool of the surfaces was necessary, and in cases when, the discharge not being oscillating of itself, the arc as soon as established was broken by the air current, thus starting the vibration at intervals in rapid succession. I have also used mechanical interrupters in many ways. To avoid the difficulties with frictional contacts, the preferred

23

plan adopted was to establish the arc and rotate through it at great speed a rim of mica provided with many holes and fastened to a steel plate. It is understood, of course, that the employment of a magnet, air current, or other inter¬ rupter, produces an-effect worth noticing, unless the self- induction, capacity and resistance are so related that there are oscillations set up upon each interruption.

I will now endeavor to show you some of the most note¬ worthy of these discharge phenomena.

I have stretched across the room two ordinary cotton covered wires, each about 7 metres in length. .They are supported on insulating cords at a distance of about 30 centimetres. I attach now to each of the terminals of the coil one of the wires and set the coil in action. Upon turn¬ ing the lights off in the room you see the wires strongly illuminated by the streams issuing abundantly from their whole surface in spite of the cotton covering, which may even be very thick. When the experiment is performed under good conditions, the light from the wires is suffici¬ ently intense to allow distinguishing the objects in a room. To produce the best result it is, of course, necessary to ad¬ just carefully the capacity of the jars, the arc between the knobs and the length of the wires. My experience is that calculation of the length of the wires leads, in such case, to no result whatever. The experimenter will do best to take the wires at the start very long, and then adjust by cutting off first long pieces, and then smaller and smaller ones as he approaches the right length.

A convenient way is to use an oil condenser of very small capacity, consisting of two small adjustable metal

24

plates, in connection with this and similar experiments. In such case I take wires rather short and set at the be¬ ginning the condenser plates at maximum distance. If the streams for the -wires increase by approach of the plates, the length of the wires is about right; if they dimin¬ ish the wires are too long for that frequency and potential. When a condenser is used in connection wTith experiments with such a coil, it should be an oil condenser by all means, as in using an air condenser considerable energy might be wasted. The wires leading to the plates in the oil should be very thin, heavily coated with some insulating com¬ pound, and provided with a conducting covering — this pref- erably extending under the surface of the oil. The conducting cover should not be too near the terminals, or ends, of the wire, as a park would be apt to jump from the wire to it. The conducting coating is used to diminish the air losses, in virtue of its action as an electrostatic screen. As to the size of the vessel con¬ taining the oil, and the size of the plates, the experimenter gains at once an idea from a rough trial. The size of the plates in oil is, however, calculable, as the dielectric losses are very small.

In the preceding experiment it is of considerable interest to know what relation the quantity of the light emitted bears to the frequency and potential of the electric im¬ pulses. My opinion is that the heat as well as light effects produced should be proportionate, under otherwise equal conditions of test, to the product of frequency and square of potential, but the experimental verification of the law, whatever it may be, would be exceedingly difficult. One

25

thing is certain, at any rate, and that is, that in augment¬ ing the potential and frequency we rapidly intensify the streams ; and, though it may be very sanguine, it is surely not altogether hopeless to expect that we may succeed in producing a practical ilium in ant on these lines. We would then be simply using burners or flames, in which there would be no chemical process, no consumption of material.

Fig. 8.— Effect Produced by Concentrating Streams.

but merely a transfer of energy, and which would, in all probability emit more light and less heat than ordinary flames.

The luminous intensity of the streams is, of course, con-

26

siderably increased when they are focused upon a small surface. This may be shown by the following experiment :

I attach to one of the terminals of the coil a wire w (Fig. 8), bent in a circle of about 30 centimetres in diameter, and to the other terminal I fasten a small brass sphere s, the surface of the wire being preferably equal to the surface of the sphere, and the centre of the latter being in a line at right angles to the plane of the wire circle and pass¬ ing through its centre. When the discharge is established under proper conditions, a luminous hollow cone is formed, and in the dark one-half of the brass sphere is strongly illuminated, as shown in the cut.

By some artifice or other, it is easy to concentrate the streams upon small surfaces and to produce very strong light effects. Two thin wires may thus be rendered in¬ tensely luminous.

In order to intensify the streams the wires should be very thin and short ; but as in this case their capacity would be generally too small for the coil — at least, for such a one as the present — it is necessary to augment the capacity to the required value, while, at the same time, the surface of the wires remains very small. This may be done in many ways.

Here, for instance, I have two plates, R R , of hard rub¬ ber (Fig. 9), upon which I have glued two very chin wires w w, so as to form a name. The w ires may be bare or covered with the best insulation — it is immaterial for the success of the experiment. Well insulated wires, if any¬ thing, are preferable. On the back of each plate, indicated by the shaded portion, is a tinfoil coating

27

4 53/,

>a/

jljX’ * f\f

t t. The plates are placed in line at a sufficient distance to prevent a spark passing from one to the other wire. The two tinfoil coatings I have joined by a conductor C, and the two wires I presently connect to the terminals of the coil. It is now easy, by varying the strength and frequency of the currents through the primary,

Fig. 9.— Wires Rendered Intensely Luminous.

to find a point at which the capacity of the system is best suited to the conditions, and the wires become so strongly luminous that, when the light in the room is turned off the name formed by them appears in brilliant letters.

It is perhaps preferable to perform this experiment with a coil operated from an alternator of high frequency, as

28

then, owing to the harmonic rise and fall, the streams are very uniform, though they are less abundant then when produced with such a coil as the present. This experiment, however, may be performed with low frequencies, but much less satisfactorily.

Fig. 10.— Luminous Discs.

When two wires, attached to the terminals of the coil, are set at the proper distance, the streams between them may be so intense as to produce a continuous luminous sheet. To show this phenomenon I have here two circles, C and c (Fig. 10), of rather stout wire, one being about

29

80 centimetres and the other 80 centimetres in diameter. To each of the terminals of the coil I attach one of the circles. The supporting wires are so bent that the circles may be placed in the same plane, coinciding as nearly as possible. When the light in the room is turned off and the coil set to work, you see the whole space between the wires uniformly filled with streams, forming a luminous disc, which could be seen from a considerable distance, such is the intensity of the streams. The outer circle could have been much larger than the present one ; in fact, with this coil I have used much larger circles, and I l ave been able to produce a strongly luminous sheet, covering an area of more than one square metre, which is a remarkable effect with this very small coil To avoid uncertainty, the circle has been taken smaller, and the area is now about 0.43 square metre.

The frequency of the vibration, and the quickness of succession of the spaiks between the knobs, affect to a marked degree the appearance of the streams. When the frequency is very low, the air gives way in more or less the same manner, as by a steady difference of potential, and the streams consist of distinct threads, generally mingled with thin sparks, which probably correspond to the successive discharges occurring between the knobs. But when the frequency is extremely high, and the arc of the discharge produces a very loud but smooth sound — showing both that oscillation takes place and that the sparks succeed each other with great rapidity — then the luminous streams formed are perfectly uniform. To reach this result very small coils and jars of small capacity should be used, I

30

take two tubes of thick Bohemian glass, about 5 centi¬ metres in diameter and 20 centimetres long. In each of the tubes I slip a primary of very thick copper wire. On the top of each tube I wind a secondary of much thinner gutta-percha covered wire. The two secondaries I connect in series, the primaries preferably in multiple arc. The tubes are then placed in a large glass vessel, at a distance of 10 to 15 centimetres from each other, on in¬ sulating supports, and„ the vessel is filled with boiled out oil, the oil reaching about an inch above the tubes. The free ends of the secondary are lifted out of the oil and placed parallel to each other at a distance of about 10 cen¬ timetres. The ends which are scraped should be dipped in the oil. Two four-pint jars joined in series may be used to discharge through the primary. When the necessary ad¬ justments in the length and distance of the wires above the oil and in the arc of discharge are made, a luminous sheet is produced between the wires which is perfectly smooth and textureless, like the ordinary discharge through a moderately exhausted tube.

I have purposely dwelt upon this apparently insignificant experiment. In trials of this kind the experimenter arrives at the startling conclusion that, to pass ordinary luminous discharges through gases, no particular degree of exhaus¬ tion is needed, but that the gas may be at ordinary or even greater pressure. To accomplish this, a very high fre¬ quency is essential; a high potential is likewise required, but this is a merely incidental necessity. These experi¬ ments teach us that, in endeavoring to discover novel methods of producing light by the agitation of atoms, or

31

molecules, of a gas, we need not limit our research to the vacuum tube, but may look forward quite seriously to the possibility of obtaining the light effects without the use of any vessel whatever, with air at ordinary pressure.

Such discharges of very high frequency, which render luminous the air at ordinary pressures, we have probably often occasion to witness in Nature. I have no doubt that if, as many believe, the aurora borealis is produced by sudden cosmic disturbances, such as eruptions at the sun’s surface, which set the electrostatic charge of the earth in an extremely rapid vibration, the red glow observed is not con¬ fined to the upper rarefied strata of the air, but the dis¬ charge traverses, by reason of its very high frequency, also the dense atmosphere in the form of a glow , such as we or¬ dinarily produce in a slightly exhausted tube. If the fre¬ quency were very low, or even more so, if the charge were not at all vibrating, the dense air would break down as in a lightning discharge. Indications of such breaking down of the lower dense strata of the air have been repeatedly observed at the occurrence of this marvelous phenom¬ enon ; but if it does occur, it can only be attributed to the fundamental disturbances, which are few in number, for the vibration produced by them would be far too rapid to allow' a disruptive break. It is the original and irregular impulses which affect the instruments ; the superimposed vibrations probably pass unnoticed.

When an ordinary low frequency discharge is passed through moderately rarefied air, the air assumes a purplish hue. If by some means or other we increase the intensity of the molecular, or atomic, vibration, the gas changes to

32

a white color. A similar change occurs at ordinary press¬ ures with electric impulses of very high frequency. If the molecules of the air around a wire are moderately agitated, the brush formed is reddish or violet ; if the vibration is rendered sufficiently intense, the streams become white. We may accomplish this in various ways. In the experi¬ ment before shown with the two wires across the room, I have endeavored to secure the result by pushing to a high value both the frequency and potential ; in the experiment with the thin wires glued on the rubber plate I have con¬ centrated the action upon a very small surface — in other words, I have worked with a great electric density.

A most curious form of discharge is observed with such a coil when the frequency and potential are pushed to the extreme limit. To perform the experiment, every part of the coil should be heavily insulated, and only two small spheres — or, better still, two sharp-edged metal discs ( d d, Fig. 11) of no more than a few centimetres in diameter — should be exposed to the air. The coil here used is immersed in oil, and the ends of the secondary reaching out of the oil are covered with an air-tight cover of hard rubb( r of great thickness. All cracks, if there are any, should be carefully stopped up, so that the brush discharge cannot form anywhere except on the small spheres or plates which are exposed to the air. In this case, since there are no large plates or other bodies of capacity attached to the terminals, the coil is capable of an extremely rapid vibration. The potential may be raised by increasing, as far as the experimenter judges proper, the rate of change of the primary current. With a coil not widely

33

differing from the present, it is best to connect the two pri¬ maries in multiple arc; but if the secondary should have a much greater number of turns the primaries should pref¬ erably be used in series, as otherwise the vibration might be too fast for the secondary. It occurs under these con¬ ditions that misty white streams break forth from the

Fig. 11.— Phantom Streams.

edges of the discs and spread out phantom-like into space. With this coil, when fairly well produced, they are about 25 to 30 centimetres long. When the hand is held against them no sensation is produced, and a spark, causing a shock, jumps from the terminal only upon the hand being brought much nearer. If the oscillation of the primary

current is rendered intermittent by some means or other, there is a corresponding throbbing of the streams, and now the hand or other conducting object may be brought in still greater proximity to the terminal without a spark being caused to jump.

Among the many beautiful phenomena which may be produced with such a coil I have here selected only those which appear to possess some features of novelty, and lead us to some conclusions of interest. One will not find it at all difficult to produce in the laboratory, by means of b, many other phenomena which appeal to the eye even more than these here shown, but present no particular feature of novelty.

Early experimenteis describe the display of spaiks pro¬ duced by an ordinary large induction coil upon an insulat¬ ing plate separating the terminals. Quite recently Siemens performed some expeiiments in which fine effects were ob¬ tained, which were seen by many with interest. No doubt large coils, even if operated with currents of low frequen¬ cies, are capable of producing beautiful effects. But the largest coil ever made could not, by far, equal the magnifi¬ cent display of streams and sparks obtained from such a disruptive discharge coil when properly adjusted. To give an idea, a coil such as the present one will cover easily a plate of 1 metre in diameter completely with the streams. The best way to perform such experiments is to take a very thin rubber or a glass plate and glue on one side of it a nar¬ row ring of tinfoil of very large diameter, and on the other a circular washer, the centre of the latter coinciding with that of the ring, and the surfaces of both being preferably

35

equal, so as to keep the coil well balanced. The washer and ring should be connected to the terminals by heavily insu¬ lated thin wires. It is easy in observing the effect of the capacity to produce a sheet of uniform streams, or a fine network of thin silvery threads, or a mass of loud brilliant sparks, which completely cover the plate.

Since I have advanced the idea of the conversion by means of the disruptive discharge, in my paper before the American Institute of Electrical Engineers at the begin¬ ning of the past year, the interest excited in it has been considerable. It affords us a means for producing any po- * tentials by the aid of inexpensive coils operated from or¬ dinary systems of distribution, and — what is perhaps more appreciated — it enables us to convert currents of any fre¬ quency into currents of any other lower or higher fre¬ quency. But its chief value will perhaps be found in the help which it will afford us in the investigations cf the phenomena of phosphorescence, which a disruptive dis¬ charge coil is capable of exciting in innumerable cases where ordinary coils, even the largest, would utterly fail.

Considering its probable uses for many practical pur¬ poses, and its possible introduction into laboratories for scientific research, a few additional remarks as to the con¬ struction of such a coil will perhaps not be found super¬ fluous.

It is, of course, absolutely necessary to employ in such a coil wires provided with the best insulation.

Good coils may be produced by employing wires covered with several layers of cotton, boiling the coil a long time in pure wax, and coohng under moderate pressure. The ad-

vantage of such a coil is that it can be easily handled, but it cannot probably give as satisfactory results as a coil im¬ mersed in pure oil. Besides, it seems that the presence of a large body of wax affects the coil disadvantageous^, whereas this does not seem to be the case with oil. Perhaps it is because the dielectric losses in the liquid are smaller.

I have tried at first silk and cotton covered wires with oil immersion, but I have been gradually led to use gutta¬ percha covered wires, which proved most satisfactory. Gutta-percha insulation adds, of course, to the capacity of the coil, and this, especially if the coil be large, is a great disadvantage when extreme frequencies are desired ; but, on the other hand, gutta-percha will withstand much m ore than an equal thickness of oil, and this advan'age should be secured at any price. Once the coil has been immersed, it should never be taken out of the oil for more than a few hours, else the gutta-percha will crack up and the coil will not be worth half as much as before. Gut a-percha is prob¬ ably slowly attacked by the oil, but after an immersion of eight to nine months I have found no ill effects.

I have obtained in commerce two kinds of gutta-percha wire: in one the insulation sticks tightly to the metal, in the other it does not. Unless a special method is followed to expel all air, it is much safer to use the first kind. I wind the coil within an oil tank so that all interstices are filled up with the oil. Between the layers I use cloth boiled out thoroughly in oil, calculating the thickness according to the difference of potential between the turns. There seems not to be a very great difference whatever kind of oil is used ; I use paraffine or linseed oil.

To exclude more perfectly the air, an excellent way to proceed, and easily practicable with small coils, is the fol¬ lowing : Construct a box of hard wood of very thick boards which have been for a long time boiled in oil. The boards should be so joined as to safely withstand the external air pressure. The coil being placed and fastened in position within the box, the latter is closed with a strong lid, and covered with closely fitting metal sheets, the joints of which are soldered very carefully. On the top two small holes are drilled, passing through the metal sheet anci the wood, and in these holes two small glass tubes are inserted and the joints made air-tight. One of the tubes is connected to a vacuum pump, and the other with a vessel containing a sufficient quantity of boiled-out oil. The latter tube has a very small hole at the bottom, and is provided with a stop¬ cock. When a fairly good vacuum has been obtained, the stopcock is opened and the oil slowly fed in. Proceeding in this manner, it is impossible that any big bubbles, which are the principal danger, should remain between the turns. The air is most completely excluded, probably better than by boiling out, which, however, when gutta-percha coated wires are used, is not practicable.

For the primaries I use ordinary line wire with a thick cotton coating. Strands of very thin insulated wires properly interlaced would, of course, be the best to employ for the primaries, but they are not to be had.

In an experimental coil the size of the wires is not of great importance. In the coil here used the primary is No. 12 and the secondary No. 24 Brown & Sharpe gauge wire ; but the sections may be varied considerably. It would only

imply different adjustments ; the results aimed at would not be materially affected.

I have dwelt at some length upon the various forms of brush discharge became, in studying them, we not only ob¬ serve phenomena which please our eye, but also afford us food for thought, and lead us to conclusions of practical importance. In the use of alternating currents of very high tension, too much precaution cannot be taken to prevent the brush discharge. In a main conveying such currents, in an induction coil or transformer, or in a condenser, the brush discharge is a source of great danger to the insulation. In a condenser especially the gaseous matter must be most carefully expelled, for in it the charged surfaces are near each other, and if the potentials are high, just as sure as a weight will fall if let go, so the insulation will give way if a single gaseous bubble of some size be present, whereas, if all gaseous matter were carefully excluded, the condenser would safely withstand a much higher difference of potential. A main conveying alternating currents of very high tension may be injured merely by a blow hole or small crack in the insulation, the more so as a blowhole is apt to contain gas at low pressure; and as it appears almost impossible to completely obviate such little imperfections, I am led to believe that in our future distri¬ bution of electrical energy by currents of very high ten¬ sion liquid insulation will be used. The cost is a great drawback, but if we employ an oil as an insulator the dis¬ tribution of electrical energy with something like 100,000 volts, and even more, become, at least with higher frequen¬ cies, so easy that they could be hardly called engineering

(I .

feats. With oil insulation and alternate current motors transmissions of power jcan_be effpcted with safety and upon an industrial basis at distances of as much as a thou¬ sand miles. \