of Edinburgh , Session 1877 - 78 . 
633 
magnetic changes is partially at least due to the molecular distur- 
bances 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. 
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 
(tig. 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 exceed- 
ingly delicate, and the up and down motion 
of its slender convolutions is most easily ex- 
cited. 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 §-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 
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