cause the current is limited-not sufficient to volatilize the copper. That brings up another feature, namely, that Mr. Reid's paper and the whole discussion have dealt with one kind of commutation only, which, however, is not the only kind, because a very large class of machines, the arc light machines, commutate in an entirely different manner. With them it is neither resistance commutation nor magnetic commutation, or, as it is perhaps preferable to call it, magnetic field commutation; but a different phenomenon, which would take too long to discuss here, but which might very properly form the basis of a separate paper. It takes place in open coil arc machines, alternating current rectifiers and self-exciting synchronous motors or alternators. Thus, you see, the commutation discussed in this paper is not the only kind of commutation which exists.

DR. WM. E. GEYER: I should like to ask the gentlemen who have discussed this matter, whether in their treatment they have considered another electromotive force which I think has not been mentioned. I have reference to thermo-electric electromotive force. That we all know is very small, and we ordinarily despise it. But in this case where the junction of two similar metals is at the melting point of copper, and the other ends are relatively cold, I imagine we might get quantities worth considering.

THE CHAIRMAN:-I may answer that question in the negative, in so far as I cannot see how you can get the effect of thermoelectromotive force, since having two couples in series, whatever electromotive force you get on the positive brush must be opposed by an opposite electromotive force at the negative brush, so the result must be zero, I should think.

DR. GEYER-I have reference to the action at one of the brushes, not to the machine as a whole, but to the actions around a single brush.

THE CHAIRMAN:-I do not know of any tests made in this respect.

If there is no further discussion on this subject we will proceed to the second part of the programme, an exhibition of "Wireless Telegraphy." But I would suggest that in view of the very interesting paper, a vote of thanks to Mr. Reid would be proper.

MR. DUNN:-May I have the pleasure of moving a vote of thanks from the INSTITUTE to Mr. Reid for his very able paper.

THE CHAIRMAN:-The motion is carried. I thank Mr. Reid here in the name of the INSTITUTE, for his very interesting and valuable paper which I am sure will be of importance and value in our TRANSACTIONS even after years.

The second part of the programme is now in order, an exhibition of "Wireless Telegraphy." I will call upon Dr. Kennelly

to take the Chair.

WIRELESS TELEGRAPHY.

December 15, 1897.

MR. W. J. CLARKE:-Mr. Chairman and gentlemen: Having been requested this evening to give you an exhibition of the Marconi apparatus, I am pleased to be able to say that I am in a position to do so. I think perhaps it might be well to give you a short explanation of the apparatus which we use. In the first place, we have here on the front of the base, a Marconi coherer consisting of a small glass tube, fitted with silver plugs connected to platinum wires at the end. These plugs are very close together, about the centre of the tube, the intervening space being filled in with nickel filings. I have found that it is entirely unnecessary to either exhaust or seal the tube. I also find that we can use almost any kind of metal for the filings. When the cohesion is examined under the microscope, I find that what we have to provide for is the proper size of the filings, and not the proper kind of metal. This coherer is placed in series with this relay which is of about 1200 ohms resistance, and made extremely sensitive, more so than the ordinary Morse relay. In series with it, also, are two cells of small dry battery placed in the base. These are arranged so that we can use either one cell or two, the object of this being that when the cells are new, we can use only one in order to prevent corrosion of the filings where they cohere. As the cells grow older we simply throw our a switch, and use two cells. This sounder is a 20-ohm instrument. It is placed in multiple with this cohering apparatus consisting of a 20-ohm vibrator placed in the base. The local battery consists of three cells of dry battery arranged as with the main battery, so that we can either use one, two or three of the cells. This Morse key is simply used in order that we may see that the apparatus is in proper condition. Pressing this key short-circuits the coherer. Our transmitter consists of an eight-inch induction coil. A much smaller one, though, is all that is necessary for a short distance. This coil has its secondary terminals connected with two brass balls, each an inch and a half in diameter. These balls are brought into close proximity, in fact in most cases touching two large brass balls, four inches in diameter. The large balls are securely cemented in the ends of a rubber tube, and the distance between them on the inside of the tube is about onethirty-second of an inch. The space between the balls in the tube is filled with the purest quality of vaseline oil, and the moment that we close the circuit through the primary of the coil the electric waves generated strike the coherer, the filings cohere, and the resistance is sufficiently reduced to operate the relay. Sometimes we have trouble with this particular instrument, not on account of the system being imperfect, but on account of the fact that it is the first piece of portable apparatus

that I know of built in this country, and of course you understand that the first piece of apparatus is liable to imperfection. I think now that everything is in proper shape, and I will proceed to the other room and close the circuit of the primary of induction coil, and I think you will see the receiver respond, and after doing this we will close the doors and work the receiver through the glass doors.

[Apparatus shown in operation.]

THE CHAIRMAN:-[Dr. Kennelly]. Gentlemen, you have witnessed an interesting exhibition, and I am sure that many must be desirous of asking Mr. Clarke some questions upon the difficulties he has had to encounter in making this instrument, and the various matters he has found necessary to take into account. I am sure that if he is as ready to answer as he has been ready to describe the apparatus that he will respond.

MR. GANO S. DUNN:-I should like to ask whether the circuit. remains closed while the shower of sparks is passing, and also, if Mr. Clarke would describe the action of the coherer.

MR. CLARKE:-I would like to say that with proper adjustment, the circuit remains closed as long as the primary circuit of the induction coil is closed, and as long as the vibrator of the induction coil is in action. Unfortunately the adjustment which we have on our decohering apparatus in this instrument is not sufficiently under control, so that I cannot get the fine adjustment necessary for transmitting intelligible Morse signals, but this is simply a question of proper adjustment. Now with regard to the coherer, as I said before, it simply consists of two conductors separated by a very short interval, this space being filled with metallic filings. The filings are of such a size and the distance apart of the conductors is such, that while the filings are lying loose, the resistance through the tube is very high indeed. But the moment that the filings cohere, the resistance is very much reduced, so much so that the current in series with the relay is able to pass to a sufficient extent to operate the relay. I may say that I have been so very busily engaged in getting a smaller and less expensive set of apparatus ready for the market, that I have not had an opportunity to experiment very largely with the question of the distance, but I expect during the next week to be able to accomplish something in that direction.

MR. DUNN:-I have done a good deal of telegraphing and am familiar with the frequency requisite to get all the signals in. For instance, when the operator is sending very rapidly, his "e" dot, by the time it reaches the other end may be only a very small fraction of a second. I noticed the frequency of your coil was readily observable, I could almost count it by beats, and I wanted to inquire how long one discharge of the coil would cause the filings to cohere, and hold the circuit closed.

MR. CLARKE:--The coherer, when in proper shape, in which this hardly is, is so very sensitive that it is not necessary to have

a vibrator on the coil at all. It is simply necessary to have a Morse key. The coherer responds the moment you close the circuit, only in this case, of course, it simply coheres and decoheres instantly, and the circuit of the sounder is closed and opened.

MR. DUNN:-If when you close your key on the induction coil that causes the filings to cohere, and then decohere immediately, your sounder here would make its down click, and be followed by its up click when the key in the other room has not risen. MR. CLARKE:-Yes.

MR. DUNN:-Then you would have to use some different system from the ordinary Morse for sending.

MR. CLARKE:-No, because when we use the vibrator on the coil, and everything is in proper adjustment, the filings cohere as long as our coil is working, and decohere the moment we stop it by opening our key.

MR. DUNN:-The vibrator frequency is about 800 a minute.

MR. CLARKE:-Well, I may say that the coil we are using is not the coil that we use for quick signaling. We use a Tesla coil for that purpose, a very high-frequency coil.

THE CHAIRMAN:-I might suggest for the benefit of some who may not be familiar with the subject, that if you sketch the outline of the connection on the board it may help.

MR. CLARKE:-I will do that, I might say that at some future time I would be very glad to show you the stereopticon diagrams on the sheet, of the different classes of this apparatus we are making up. It is a little difficult to explain it on the blackboard, but I will do the best that I can. Here we have the coherer a somewhat enlarged. These B B' are the silver plugs. The filings are between the ends of those plugs. The main battery B2 is connected to one end of the coherer. The other ter

minal of the battery is connected to one terminal of the 1200ohm relay R. The other terminal of the relay is connected to the coherer. Now when the apparatus is in its normal condition, the relay is so adjusted and the resistance of the filings is such, that the current from the battery will not pass to a sufficient extent to operate the armature of the relay, but when the waves from the transmitter strike the filings and they cohere to each other, and also to the silver plugs, as examination under the microscope shows that they do, the resistance is so reduced as to allow sufficient current to pass from this battery to operate the relay. The moment the relay operates, it pulls up its armature, making the contact c. The moment this contact is made, the current from the local battery L traverses the wire w to the armature of the relay R', from the armature across the contact to the screw s, back to the sounder D and back again to the battery. Now once the filings have responded to the waves and cohere, they will remain colering unless we have some means of decohering them. In order to accomplish this, we have a vibrating hammer which strikes the tube. This vibrator is placed in multiple circuit with the sounder. They are both of comparatively high resistance.

MR. EDWARD DURANT:-I would like to ask what is the greatest distance at which you can get communication?

MR. CLARKE: I may say that I have not tried the instru ments at very long range. I have not had either the time or the opportunity. But I understand from the reports coming from the other side, that Mr. Marconi has had no difficulty in transmitting intelligible signals twelve miles.

MR. MACGREGOR :-I should like to ask one or two questions. The first is, does it make any difference as to what the relative position of the tubes containing the filings is? Is the position you have it in all essential?

MR. CLARKE:-I have not found that it is.

MR. MACGREGOR -The second thing I want to ask is, whether there is any fixed relation between these two pieces of apparatus. In other words, would any other induction coil produce the same effect as this one, or are these two instruments related so as to form a pair? If intelligence transmitted from one piece of apparatus could be read by any one of a hundred machines, this method of telegraphy would have its disadvantages.

MR. CLARKE:-In the first place, I have not found using the short ranges over which I have tried the apparatus, that it makes any difference in what position either the receiver or transmitter is placed. In regard to their being any tune, as we may call it, between the transmitter and receiver, I may say that Marconi claims that it is absolutely necessary to have the transmitter and the receiver in what we might call synchronism. In order to accomplish that, he takes two strips of copper, each about half an inch in width, and attaches them to the end of his tube, one