portion to the conductivity of each. If it now be accepted that the sounding action be like heat proportionate to the square of the current strength, the apparent discrepancy is accounted for. Suppose the conductivity of silver to be ten times greater than that of German silver, then in the silver plate a tenfold greater current would be induced, and would have in consequence one hundredfold greater sounding power than that of the German silver in the same resistance; but as the resistance of the German silver is ten times greater than that of the silver, there would still be a tenfold louder sound in the silver than in the German silver, quite a sufficient margin to remove the discrepancy. In addition to the action within the plates here discussed, there may of course also be the external push-and-pull, as with the iron disc. In wires the length involved has little effect on the sound; for though a long stretch of wire does sound louder than a short one, the difference is by no means in keeping with the respective lengths. In thick wires no sound is got. I tried in vain with the strongest current I could conveniently use to get a sound out of a No. 14 copper wire.

Now here at least it will be admitted that we have a molecular action. It may be replied that the earth's magnetism has something to do with it. To show that it has not, we may try the somewhat inelegant feat of holding the end of the thin iron wire in the teeth, and turning it in every possible direction, when we shall find that the loudness of the ticking has no relation to direction. Again, when the string of the telephone is tied to the wire and made to go round it in a circle, we shall find no maximum or minimum points. This then is a telephonic action absolutely free from external push and pull, proceeding undoubtedly from molecular disturbance in the wire. That the action is magnetic in some way is evident from the exceptional position of iron in the series of metals. De la Rive held that the sounding action of an iron wire, when the seat of a discontinuous current was almost identical with, and must be traceable to the same ultimate cause as, that it displays when placed in a magnetising helix excited by a similar current. The parallelism between the wire telephone and the ordinary electric telephone is so striking that no theory of the latter can be held to be complete without it includes the former. It seems to suggest that the sounds we hear in plates and rods under sudden electric or

magnetic changes is partially at least due to the molecular disturbances set up by induced momentary currents. Be that as it may,

surely no one would be inclined to say that the ticking of a Bell telephone under a series of momentary currents is wholly mechanical, and that of a wire telephone under the same conditions wholly molecular.

Fig. 9.

After observing these sounds, my next inquiry was to ascertain how they comported themselves in a helix capable of motion. With this view I coiled about ten feet of fine iron wire (0173 inch diameter) into a spiral about inch in diameter. I suspended it (fig. 9) by a thick wire held on a stand, and arranged that its lower end should dip into a vessel of mercury. Such a spiral is exceedingly delicate, and the up and down motion. of its slender convolutions is most easily excited. When left free to itself, and there is no interruption in the mercury below, that is, no spark reaction, the motion produced by a discontinuous current of five Bunsen cells is very slight; but if a 3-inch soft iron rod be introduced so as to be clear of the sides of the spiral, the mechanical excitement of the spiral is more marked, but still. not much. But if I hold down the lower end, and it so happen that the tension and number of convolutions be sufficient, the helix

divides itself into two very active ventral segments with a node in the middle. The motion, especially at the middle of each segment, is now very considerable. But if it does not vibrate thus symmetrically, by shifting the fingers a little, a point is easily found by which at least one very active ventral segment is secured. If now to a point of maximum motion I solder a very fine copper wire, to serve as the thread of a mechanical telephone and make it come out at right angles to the motion of the spiral, it does not interfere with it. By this contrivance I secure the advantage of listening to what goes on in the wire without interfering with any mechanical magnetic effect. Profiting by the experience of the fork, I found that a fine wire could hang loosely, and yet convey vibrations to

the telephone. Fine iron wire answers very well, but in this case it is inapplicable, as it sets up a clearly audible action on its own account. When the ear is now applied to the telephone the ticking that you would hear in the wire, if it were straight, is heard distinctly, and is the only distinct sound, for the up and down motion of the convolution only produces a slight jingling in the connecting wire, and possibly also in the coil. When the convolution is held tight in the fingers the ticking goes on if anything more distinctly than before. Whether the mechanical motion by the increased fixity of the helix is converted into louder ticking, could not be decided by the difference. Here we have two vibrations quite distinct from one another, the ticking in the wire and the mechanical motion due to the mutual electro-magnetic action between the rod and the coil. If this last were quick enough it would also be telephoned, but the rate of vibration being below that of musical frequency it is nothing but inaudible motion. In this helix action I would submit we have a dissected view of all receiving telephonic action-a vibration in the body clearly of molecular origin, and another of the body of a push-and-pull kind. The latter may be stopped or nearly so, but not the former. From its internal origin it is bound to make itself good, and when the body is held in the grasp of the most rigid substance it only propagates in it the minute vibrations which no elastic matter can stop. In the vibrations of coils, cores, or plates, the same thing holds. Molecular vibration is present in them all, and how far mechanical vibration chimes in in unison depends entirely on the ease with which such can be produced.

There is apparently a marked difference between these two vibrations. The one can make itself good acoustically by one impulse, the other not. I have tried in vain, while holding taut the thread of a mechanical telephone without letting the fingers slip, to produce, by a sudden pull or let go, the tick that resounds in iron or a conductor when a current suddenly begin or ends, and I could only do so by letting the string slip the slightest degree, so as to set up a short series of vibrations. Each galvanic or magnetic tick or tap may not, in the first instance, be more than a simple shock; but so sudden is it that the particles concerned do not recover at once, but continue vibrating for an infinitesimal time, and hence

the pitch or musical sound that generally accompanies it. When, however, a prolonged vibration is difficult, and the impulses brace each other up by frequent repetition, we have possibly in the first case a very short series, and in the other only one vibration. Such vibrations are as capable of rendering all complex acoustic combinations as vibrations of the push-and-pull kind.

It would be a matter of mere speculation to guess how the conditions of the vibrating helix are transferred to vibrating rods or discs of iron in an intermittent magnetic field. I would only say that the same double vibration is clearly traceable in them. To illustrate this in the case of rods, I took a small coil of No. 20 wire, 2 inches in diameter and about an inch high with a hollow axis of inch, and sent the interrupted current of a five Bunsen cell battery through it. Inside the axis I put a soft iron pin 2 inches long and inch square (fig. 10). To the upper end a fine copper wire was soldered to act as the thread of a telephone. When rightly placed the pin supported itself in the hollow, and kept dancing up and down symmetrically without much friction against the inside. of the bobbin hollow. Here the mechanical motion was not so clearly eliminated as in the case of the helix; yet the ticking was heard, and it alone, when the motion of the pin was stopped by the hand. The impossibility of stopping the ticking of the pin was

Fig. 10.

shown by securing its ends between the jaws of a vice and making it as tight as possible, when the vice itself took up the tale of electric interruption, and made itself heard all round. A curious change was observed in long iron rods when this coil was placed round the middle of them and when shifted to the end. In the former position the sound was a stuccato rendering of the longitudinal note of the rod, and in the latter this sound was lost in a dull tapping. In

the latter position there was a pronounced tendency to push and pull.

I adopted a similar arrangement with the vibrating disc of the telephone. To the middle of one I cemented an india-rubber tube to act as a yielding handle, and the paper telephone wire was soldered near the middle. The disc was made to move in front of a telephone core excited by the coil just named. The movements of the disc were very violent, and made in all directions, making the connecting string jingle loudly, so that the isolation of the ticking sound was not so satisfactory as in the two previous cases. Still it was heard, and when the motions of the disc were kept in one direction its loudness did not grow with the extent of the motion. With a fine coil and a water cell the ticks were distinct enough; but the mechanical displacement was too small to yield the required comparison. The impossibility of stopping this ticking was illustrated in the following way:-A ferrotype plate was held tightly between two thin pieces of plate glass, the space between being filled up with sealing wax so that the whole was a solid mass of glass and wax. This was brought near a core excited by a water cell, when the sound was loudly rendered. Another illustration to the same effect was that of cementing by sealing wax the ferrotype plate of a telephone to a disc of thick microscopic glass, and putting this in the telephone with the glass side to the ear or mouth. Its articulate functions, though much impaired, still continued.

Lastly, to test whether the tick in a coil was due to electric conduction, I screwed a pin into the core of the telephone so as to act as a prolongation of the core; round this I placed a coil of fine wire, to which the string of a paper telephone was attached. There was no ticking heard so long as the circuit of the coil was broken; but the moment it was closed the ticking began. The coil was, of course, clear of the core. At the same time, however, there was mechanical action between the coil and core, illustrating the difficulty in such cases of determining by direct observation whether the single mechanical pull may not also make itself heard.

In conclusion, I would say, by way of summing up the evidence of this paper, that at the sending station the evidence of molecular action, though suggestive, is by no means conclusive; while at the receiving station the existence of molecular as well as mechanical