has described here, brought out by them on the other side. I can scarcely agree with the remarks of Mr. Willyoung in regard to the advantage of that instrument. I think if he makes careful experiments and careful tests of different forms of coil, he will find that Messrs. Ayrton and Perry have made a fundamental mistake on that. That type of coil is not as good as the other type of coil. It has some advantages and it has some disadvantages. I read the paper twice or three times over, because it dealt with a branch of work to which I had given a great deal of attention, and it seems to me that the author has overlooked a good many important matters. In practical work an index is accurate enough for an instrument that is to be used to measure electromotive force or current strength. In making that statement I make it because I know it to be a fact, that with a very slight increase in the length of the index over our regular type of instrument it is perfectly possible to get an instrument that will work within of one per cent. If a man can standardize an ins.rument within that limit he is doing remarkably good work; I think a good deal better work than it is possible to do generally.

The little voltmeters and ammeters which he mentions are not the ones I refer to. Those are capable of being used to indicate correctly to within one-tenth of one per cent. I think they will be found to be, if not abused, correct to within about one-fifth of one per cent., and where they are called for we can make them correct within one-tenth of one per cent. and a man with a little experience can read to one-tenth of those divisions with accuracy. I find no difficulty in it. I have compared them with very delicate instruments and I found no serious error in estimating the tenths at all. It must not be overlooked that with a mirror and scale, the working angle is generally very small. A mirror and scale instrument is unquestionably an admirable thing for the adjustment of resistance. But for the measurement of current strength and electromotive force I think it is wholly unnecessary. With the mirror and scale as usually employed the working angle is very small. On the other hand, in a portable instrument or instrument with indexes, the working angles are as large as 90° and can be made even more if necessary. So if you take a mirror and scale you have either got to use a curve scale and increase the working angle to get very much greater sensibility or accuracy, and that never can be quite equal to the index, due to the fact that you cannot move over one-half the angle with the mirror that you can with the index.

I agree with what Mr. Willyoung says in regard to the portable instrument shown in this case. I think the author has a little difficulty about how much travel he has with the type of instrument he shows here. The coil is about three inches long and very nearly two inches in diameter. How to carry such a coil as that upon a pair of pivots and to expect to use it with a pair of springs which would ensure extreme sensibility and at the

same time also use it with a mirror, I tell you it requires a good deal of courage to take that kind of a job up. If any try it they will find that they have got to throw it away and start all over again.

One of the serious difficulties in making these portable instruments is to get the weight on the pivot just as minute as possible. In our earlier models we did the best we could in this direction. We made the coil just as light and of as fine wire as we could, considering the mechanical skill we had at our command and the means of disposal in an instrument of that character, for those portable instruments require workmanship of an exceedingly high degree of accuracy. In the original coil the copper weighs somewhere, as I remember now, about 45 grains. The coil on the instrument made to-day weighs four grains; two-thousanths of an inch wire. We tried to get it to one-thousandth, but could not cover it. We got down to two-thousandths and succeeded very well there. The moving coil, pivot and all, without the index, weigh about 10 grains, or two-thirds of a gramme. The moving coil and frame alone weigh about seven grains. The needle weighs about three grains. It is made of aluminium and made in the manner described in this paper and has been made so for many years. I think there are vastly better instruments obtainable than the one shown here.

In regard to the use of the mirror, I would like to say that we have employed the mirror on one of our instruments and I have brought one here. It was made, I think, some time in 1879 or 1880. The scheme of supporting the mirror is a great deal better one than that described in the paper. The core is not cut away in the manner described, which certainly is a very detrimental thing. A better form is one which was made about a year or a year and a half ago, shown in the other model. There is no necessity for cutting away or removing such a large portion of the core as is shown in the form described in the paper.

The

DR. DUNCAN :-Mr. Willyoung and Mr. Weston seem to differ as to the best shape of core. Mr. Willyoung favors having it very long and narrow. Mr. Weston thinks that it should be square. There is the question, of course, if the induction is the same in both cases, that the square coil is much better. turning moment depends on the change in the number of lines of force for a given angle of deflection, and with a square coil that change would be greater for a given length of wire than with the other. If you have a long coil and no iron core, I imagine you can get more induction through a long coil, but if the induction is the same, as I say, the square coil should be better. If the core is used, then I think Mr. Willyoung is unquestionably wrong and that Mr. Weston's contention is true-that you have the greater turning moment with the square coil.

MR. WILLYOUNG:-I am willing to agree with Dr. Duncan in that respect. My remarks on the galvanometer, of course, were

applied to the galvanometer as used as an instrument of precision, that is to say, in work which requires high sensibility. There is no question at all that for Mr. Weston's purposes the square coil is very much better in every respect, mechanically, electrically and every other way. But where extreme sensibility is required I think Mr. Weston will admit that it cannot be obtained. I mean the kind of sensibility that is required in very exact work, with the Wheatstone bridge and measurements of that kind. It cannot be obtained by coils suspended on pivots, and torsion produced by springs. It can only be obtained, as far as I know, by the use of a suspension, and it cannot be obtained perfectly enough even with that. It is not possible at present to produce a D'Arsonval galvanometer which is suitable for testing the insulation of cables unless the deflection method is abandoned for the condenser method. In the orthodox method of testing cables you require a very sensitive instrument which has a constant of a good many megohms and you get small scale divisions, of course, because you require a sensitive instrument and you could not get large scale divisions. You have not got the sensibility for a large angle. However, at the present time, I do not know of any D'Arsonval galvanometer that has ever been made that is sensitive enough for the application of the deflection method to the testing of cables or that begins to compare in sensibility with the Thomson galvanometer. Professor Ayrton says it does, and he has some mathematical arguments with which to support his views. But in actual practice it does not. I have never seen one that does. I have never seen any one that has seen one that does. I have not been able to come anywhere near to it myself. What I have to say about the galvanometer applies entirely to its use as a very delicate instrument. For commercial purposes, certainly, it would be impossible to improve on Mr. Weston's instrument. I have a great admiration for Mr. Weston's instruments, the manner in which they have been worked out, and the beautiful way in which they work. Undoubtedly the square form of coil, and his method of construction are the best in that form of instrument. But for an instrument requiring great sensibility I have found by actual experience with different instruments that I can get vastly more sensibility out of a form of galvanometer which has an elongated coil.

DR. DUNCAN:-My contention is that it would be more positive with a square coil provided the induction were the same in all cases.

MR. WILLYOUNG:-Precisely.

PROF. R. C. CARPENTER :-There is just one remark I would like to make regarding some light metals which might be of interest. During the last year we have been investigating under the direction of Dr. Thurston the physical properties of a new light metal. It is not new in any general sense, but it is new to us, and I do not think its properties are usually known. I refer

to the metal magnesium. We find that it is in many respects very much superior to aluminium. Its specific gravity is about two-thirds that of aluminium. Its strength runs from 18,000 to 22,000 pounds per square inch. We have broken a great many pieces of aluminium and never yet have found one stronger than 12,000 pounds to the square inch. Its usual strength is 8,000 to 11,000 pounds. Magnesium is very much more ductile than aluminium, and so far as we can determine, it is not oxidized to any great extent by the atmosphere, certainly not at temperatures under 125 degrees. It seems to me that in looking for light metals it is well worthy of attention. At present, of course, the price is five or six times that of aluminium.

MR. WESTON-In connection with that I would like to ask if it is possible to work it satisfactorily.

PROF. CARPENTER :-It apparently works very nicely, although we have had a good deal of difficulty in producing alloys. We can alloy it with one per cent. of copper very readily. But when we pass one per cent. we have difficulty. With aluminium we can alloy it readily, and the alloy of aluminium and magnesium has double the strength of pure aluminium.

MR. WESTON :-I asked that question because some time ago I undertook to construct an electrostatic voltmeter using a magnesium needle instead of an aluminium needle. I obtained a sheet of comparatively thin magnesium, but it was not thin enough for my purposes. It seemed impossible to work it, except by actually driving it down, just rolling it.

PROF. CARPENTER :-Possibly we have had a different class of magnesium. Our magnesium has resembled lead in its ductility. It seemed to have no well defined elastic limit. It seemed to give gradually as the force is applied, yet its ultimate strength is very great. It has a silver lustre much more like silver than that of aluminium.

[Professor Carpenter then read the following paper:]

of the American Institute of Electrical Engineers, New York, May 17, 1893. President

Houston in the Chair.

THE VARIATION IN ECONOMY OF THE STEAM ENGINE DUE TO VARIATION IN LOAD.

BY R. C. CARPENTER.

The writer recently made some tests on two classes of engines, subject to variations in load, the results of which were reported at the last meeting of the American Society of Mechanical Engineers. Since that time the matter has been given considerable attention, from a practical standpoint, and the following paper contains the results of considerations and a comparison of all the data at hand, with certain simple formulæ. The object is the production of a practical formula to express the economy of any class of engine, under any condition of loading.

In deducing the formule which are expressed later, certain fundamental principles were used as a basis for reasoning, which, so far as this paper is concerned, need be considered as resting only on an empirical basis, although the author has reason to suppose a broader foundation for the propositions which are stated. The fundamental proposition will, I think, be generally admitted. It is as follows:

I. The steam consumed by an engine may be considered as composed of two parts, one part being that which would be required to do the work in a perfect engine, which is defined here as an engine working without waste of any available heat, and whose efficiency is equal to the thermodynamical efficiency of an engine working within the same limits of temperature; the other part being that required to overcome the losses due to cylinder condensation and various wastes of the engine.

The steam required by a perfect engine, which as defined above, is one not subject to wastes, except those due to the