MR. MARKS: In the earlier stages of this lamp there was a marked tendency to explosion, and on several occasions the attendants were forced to leave the room, but after perfecting the little valve we found that we had no further trouble in that respect. Indeed, I have not known a cylinder either to break or the gas to cause any disturbance for probably six months. Prof. Nichols alluded to the character of the glass which must be used in the incandescent arc of this style. It is very true that a great deal of difficulty has been experienced in procuring a glass suitable for this purpose, but after a series of experiments one of the glass manufacturers was enabled to give us a glass of a very refractory character which has withstood all the tests of temperature. Then in regard to Prof. S. P. Thompson's remarks, I would like to state that in the early part of the paper, before he came in, mention was made of the fact that the negative carbon wasted by combustion only. He has cleared that point in his discussion and has shown why the disintegration in the negative was not at all marked, and that it was due almost entirely, if not entirely, to the reflection of the heat from the positive electrode or "roasting," if I understood him correctly. In this type of lamp there is practically no waste of the negative electrode.

MR. GEORGE W. McDONALD:-This lamp appears to me to be rather an impracticable solution of the problem of making the carbons burn for a longer time than is now the case, because I have found that material is thrown off by the arc in such quantities as to cover the inclosing globe and prevent the exit of the light.

MR. MARKS:-In regard to that point I would like to state, or rather to refer to what was said in the paper regarding the quality of the electrodes. Absolute purity of the carbon is imperative, and no doubt in the case of the experiments of the last speaker, the carbons were, to a certain extent, imperfect, and judging from the results which he obtained, I am quite sure that the electrodes had a small percentage of iron in them, inasmuch as nearly all carbons manufactured, on this side of the water, at any rate, have a small amount of iron in them; even one-tenth of one per cent. of iron in a carbon is fatal to this form on light, because the iron is thrown on the internal surface of the glass cylinder in the form of an opaque coating. I have found with these experiments, that the pressure of the gas has tended to keep the body of the carbon intact and to prevent to a large extent, the deposition of carbon on the internal surface of the chamber, but with absolutely pure carbons of the requisite structure-and that is a very important matter-the deposition is very little. I ran carbons for hours and hours without any trace of deposit on the surface, but it took many months of experimental work to get a carbon which would answer that condition. Prof. Thompson alluded to the cleaning of the globes. We have found the coatings of the globes to be a practical benefit, inas

much as it gives a more symmetrical distribution of light. A great many who have seen the light both with the coated cylinder and with the plain cylinder have preferred the former arrangement. The light seems to be softer, the luminosity appears to be greater. No exact measurements were made on this, but it is hoped in the near future we will know a little more about this phase of the subject.

The Section then adjourned to meet at 10 A. M. the following morning.

THIRD MEETING, THURSDAY, AUGUST 24TH, 1893.

DEVOTED TO A GENERAL DISCUSSION OF POWER TRANSMISSION.

The session was called to order at ten o'clock by the Chairman, Prof. Edwin J. Houston, who, after giving a brief statement of the work before the Section, introduced Professor F. B. Crocker, who spoke "On Direct Current Dynamos of Very High Potential."

DIRECT CURRENT DYNAMOS OF VERY HIGH

POTENTIAL.

BY PROF. FRANCIS B. CROCKER.

Professor of Electrical Engineering, Columbia College, New York.

Mr. Chairman and Gentlemen: This treatment of the subject is not a formal paper, being merely a note of some experimental results which I have obtained with two direct current dynamos giving currents of very high potential—that is, from 5,000 to 11,000 volts. Machines of this kind have received comparatively little attention either from scientists or practical engineers. Nevertheless, facts of great scientific and practical importance can be derived from the investigation of this class of dynamos and motors.

Considering these machines historically, the first fact that meets our attention is that there exists a general and deeplyrooted idea that direct current dynamos of very high potential are not at all practical. This unfavorable opinion is particularly strong in regard to the use of such machines for the transmission of power for any considerable distance; in fact, such a system is considered to be almost out of the question.

It is chiefly with the object of bringing this system into the discussion of power transmission that I venture to present the following facts to the Congress.

The actual historical and practical facts are that the high potential direct current machines were more extensively and successfully operated when the dynamo first came into general use about 1880 than any other type, either direct or alternate current. Furthermore, their number and size have largely increased and the voltage at which they can be practically worked has been steadily raised until we now have sixty-light are dynamos as the

standard size of large machine, generating about 3,000 volts and 10 amperes. Are dynamos of 90 or 100-light capacity are also regularly made by several manufacturers, and 100 or even 125light machines have been built. I happen to know of one station where there are four arc dynamos rated at 125 lights each, which run every night with a load of from 100 to 125 lights. These machines generate at least 5000 volts each. No great practical or other difficulty is found in operating are machines, except that of danger to persons, but this is merely due to the high potential and does not depend very much upon the type of machine or character of current. In fact, the direct is probably safer than the alternating current of the same voltage. Nevertheless, when it is suggested to use direct currents in the transmission of power, we are usually told that nothing over 1000, or at the most 2000 volts, is at all practical. Why this discrepancy between the 5000 volts which are practically used in arc lighting and the 1000 or 2000 volts that are considered the limit of such machines for power transmission? Perhaps the first answer to this question would be to say that the current is limited; that when we have 5000 volts we cannot have more than 10 amperes, and if we want more amperes we must have less volts; consequently the number of watts is limited. For example, the machines which I cited and which are in practical and successful use for arc lighting give 5000 volts electromotive force, and only 10 amperes, and consequently have a capacity of 50 kilowatts, which is a small power, comparatively speaking, for power transmission, but is sufficient for ordinary are lighting circuits. That, of course, is a fairly good explanation of the reason why such machines are not applicable to power transmission. But is there any such limit as 10 amperes to the current? I myself always look with great suspicion and doubt upon any such arbitrary limit as that. Experience has shown me that these arbitrary limits are usually imaginary. Now, it is a fact, however, that should be added to the historical consideration of the subject, that numerous attempts have been made to employ such machines for power transmission and other purposes, and it cannot be said that those attempts have been very successful; in fact, it can be said that they have generally been unsuccessful, but that is merely a negative fact.

In considering the actual construction of such machines, the first point is insulation which must be above suspicion. The

ordinary insulation resistance of one megohm is simply nothing for such machines. One megohm with 10,000 volts would allow .01 ampere to leak which would give 100 watts and that would rapidly heat and destroy the insulation. The insulation resistance should be at least 100 megohms. The next point is the commutator, because although the commutator might be considered the most important feature, as a matter of fact we must have the insulation before we can operate the machine at all, even for a few moments. The commutator, of course, must have a large number of sections. It must have a considerable thickness of mica insulation between the bars, more than is ordinarily employed; I should say about one-tenth of an inch. On the end of the commutator, where there exists the total voltage of the machine between two opposite commutator bars and the metal ring which holds the bars together, we require very much more thickness of mica insulation or some other insulation than is ordinarily given, at least inch. The next point is the material for the brushes, and I have found in that particular feature the most peculiar and important differences. In the first place, I consider that it is impossible to run an ordinary mica-insulated commutator with copper brushes at high potentials, the reason being that the film of copper which is worn off of the copper brushes by the mica is a sufficiently good conductor at very high voltage to carry many watts of current. At a low E. M. F. we do not have this difficulty, because even if we use copper brushes, the film of copper that is rubbed on to the surface of the mica would not carry a sufficient number of watts to cause any trouble, but at 10,000 volts, with a difference of potential in the neighborhood of 100 volts between adjacent bars, the film of copper, even although it is only infinitesimally thick, is sufficient to carry many watts of current.

I have found that with a potential of only 5,000 volts, which I have experimented with, copper brushes could not be used for half a minute; the copper would rub off upon the mica and immediately produce a ring of fire all around the commutator. Naturally one would use carbon brushes in such a case, because the current is small, and there is no reason why we should not use them. In regard to carbon brushes, I have also found that hard carbon is better, for the reason that it does not produce a deposit or layer of carbon on the commutator which might produce somewhat the same effect as copper, but not to the same