After Guglielmo Marconi announced his advances in distant signaling using radio -- then known as "Hertzian waves" -- he applied for a British patent in 1896, which described technical information about his system. In this review, W. J. Clarke demonstrates a spark-gap transmitter plus a coherer receiver, based on Marconi's design, for a meeting of the American Institute of Electrical Engineers in New York City.
1897 Transactions of the American Institute of Electrical Engineers, pages 607-613:


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 one-thirty-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, has "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.
apparatus diagram

    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 terminal of the battery is connected to one terminal of the 1200-ohm 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 R1, 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 cohering 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 instruments 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 on each end. These strips of copper he claims have been shortened or lengthened in accordance with the frequency of our transmitting apparatus. In order to ascertain what is the proper length, he takes a piece of glass and pastes upon it a strip of tin foil about half an inch in width, and two or three feet in length. He divides this strip in the centre with a very sharp knife. He sets his transmitter in a dark room and places this testing apparatus in front of it, at a few yards distant. He operates his transmitter, and when he does so, the same as in the experiments of Hertz, he notices small sparks passing across between the divided tin foil. He moves his tester further from the transmitter until the sparks entirely disappear. Then he cuts off, say half an inch, from each end of his tin foil and immediately the sparks reappear. He goes further, until the sparks disappear again, and he keeps on cutting down each end of his tin foil, until he finds that he is working in the other direction, and that if he cuts it any more, he simply has to go back closer to the coil in order that the sparks at this point will reappear. Then he finds he has got the proper length. Accordingly, he cuts what he calls the wings of the coherer nearly the same length as these two pieces of tin foil. I may say that in the comparatively small distance over which I have tried the apparatus, I have not found at all, or that it is it necessary to have any wings on the tube necessary to have the apparatus in any kind of synchronism, I find that some recent writers in the London Electrician bear me out in this. I see it stated that in one of the exhibitions given by Mr. Marconi, or some of his associates, that in order to show how readily the waves would go through a piece of iron, the coherer was placed in an iron box, and in order to place it there, it was necessary to remove the wings, but the coherer responded just the same.
    DR.  SAM'L  SHELDON:--I would like to ask if it is essential that you should have silver for the two electrodes.
    MR.  CLARKE:--The only object, as far as I can see, is simply to keep the points of contact clean.
    DR.  SHELDON:--Then the object of a vacuum would be to prevent the silver from oxidizing.
    MR.  CLARKE:--Yes, and I may say in this connection too, that experiments have been tried with putting the filings in hydrogen gas, but it was found that they speedily became too bright and clean, so much so that they cohered all the time. If the filings are examined under a microscope and the waves are acting upon them, it will be noticed that the moment they decohere you will see little bright spots where the particles of metal have been touching each other. Those bright spots rapidly disappear.
    MR.  C.  T.  CHILD:--I would like to ask if that decoherer is in the nature of an electro-magnetic vibrator.
    MR.  CLARKE:--Yes.
    MR.  CHILD:--How do you shield the coherer from the waves sent out from that instrument?
    MR.  CLARKE:--We do not find that it affects them in the least.
    MR.  CHILD:--I should think that those waves would affect it.
    MR.  CLARKE:--Well, Mr. Marconi claims that it does. I have not found any effect at all. Perhaps it is because I have guarded against that by cutting down the sparks at the contact with resistance coils, and also placing a resistance across the terminals of all the magnets.
    A  MEMBER:--I fancy that the resistance must be very low with those metal filings there, so low that the variation must be very small as we find at once in telephone experiments where we use metal filings. May I ask what the resistance is of that coherer?
    MR.  CLARKE:--I may say that although I have the facilities, I have not had the time to make a thorough test in that direction and I can only guess at what the resistance really is. I expect very shortly to devote very considerable time in making a thorough test.
    THE  CHAIRMAN:--It is very high, isn't it?
    MR.  CLARKE:--I hardly think so, because you will see we have a resistance in the relay of 1200 ohms, and one cell of dry battery is all that we require to operate it. In fact if we use silver filings it is difficult to get them to decohere enough to interrupt the flow of current. I should have stated that silver is the only metal I have found that is too sensitive.
    MR.  WINTRINGHAM:--How so?
    MR.  CLARKE:--If we use silver filings we find it almost impossible to adjust the apparatus so that the circuit will remain open and then close when the waves strike it. Silver filings are so very clean and bright, that they form a very low resistance across the plugs in the tube, much lower than nickel or any other metal that I have tried. I have not tried gold or any metal of that kind. I do not know how they would act.
    THE  CHAIRMAN:--If there are no further questions, I feel sure that you will endorse my recommendation that a vote of thanks be extended to Mr. Clarke for his kindness in exhibiting the apparatus, and in so patiently answering all our questions prompted by a very natural curiosity. I need hardly put that to vote. I think I am justified in extending that vote of thanks.
    MR.  ALBON  MAN:--Will the gentleman answer one question? The origin of waves seems to be provided for between the large balls in the rubber tubes. Why are not other waves generated between the small balls and the large ones, or are they generated, and do they have the same effect as the ones in the tube? I understand that sparks pass between the terminals of the small balls and the large ones.
    MR.  CLARKE:--Yes.
    MR.  MAN:--Are they not a source of electrical waves?
    MR.  CLARKE:--In regard to that I can hardly say; I have not experimented far enough. Mr. Marconi claims that it is necessary not only to use two balls four inches in diameter in the tube, but also use the other balls of the same size. Now I find that over the short distance that I have worked the apparatus, it is not necessary even to have the small balls at all. All that is necessary is to connect the terminals of the coil to the large balls, and I have used a coil of only one inch spark capacity. I feel quite positive that a coil of that size is all that is necessary for transmitting this distance.
    MR.  MAN:--I would like to know also if the diameter of the large ball and its mass, is a necessity to the transmission of the waves, or the production of the waves. Has the mass of the balls anything to do with the force of the waves?
    MR.  CLARKE:--I can only say in regard to that, that I have followed Mr. Marconi's experiments very closely. I have only so far constructed a transmitter with the four inch solid brass balls. Marconi claims that it is absolutely necessary to have solid balls for long distance transmission, and that with hollow balls, the distance over which we transmit will be less than half. My own impression is, that hollow brass spheres filled with lead, or other inexpensive metal, would be just as good, and we are going to experiment with that shortly, and also with the smaller spheres, on account of the larger spheres being very expensive and very heavy.
    MR.  MAN:--Hertz found his electrical waves produced as readily from small balls, or even any kind of a spark as from larger masses. It was for that reason that I asked the question.
    MR.  CLARKE:--Knowing the results of the experiments of Hertz, I am of the opinion myself that the size and mass of the ball does not make the difference that is claimed for it, and for that reason we are going to experiment in order to determine that accurately.
    MR.  HAMBLET:--I move a vote of thanks to Mr. Clarke for the very interesting exhibition that he has given us this evening.
    [The motion was carried, and the meeting adjourned.]