684 
Proceedings of the Royal Society 
increases with, the length. There must, of course, he a limit to 
increase in this way ; hut, unfortunately, the stock of fine wire 
obtainable in Edinburgh is neither very varied nor very extensive ; 
so that I could not examine this point farther. 
With the same wire, the effect increased with the current 
strength. The notes came out best when the current just heated 
the fine wire to a very dull red. It was beautiful then to see the 
wire burst into a bright glow when reproducing a prolonged note, 
especially a high one. This glow is sometimes so strong that the 
wire softens and breaks under the tension. When the wire is 
shielded from air currents, the glow can be seen to follow the swell 
of the music, and, with a wire 10 or 12 Cm. long, the motion of the 
end could be seen quite distinctly keeping time with the swell of 
the music. 
Heating the fine wire externally by means of a lamp increases 
the effect slightly, and cooling with water or a blast of air seemed 
to produce the opposite effect, but only slightly in the case of very 
thin wires. 
[Added May 18 tli . — Since the above experiments were made, my 
attention has been directed to a paper by Dr Ferguson (Proc. Roy. 
Soc. Edin., 1877-78, p. 628), in which he anticipates to a certain 
extent, the main experiment above described. It is true that he 
has not applied his apparatus to the transmission of music or articu- 
late speech as I have done, but he makes the practically very im- 
portant step of attaching a mechanical telephone to the wire convey- 
ing a varying current, and thereby renders the observation of the 
sounds of De La Rive both easy and certain. He has also given 
the very important result that these sounds may be caused by 
currents of very small total heating effect, such as induction 
currents. This I have since verified in certain cases. 
Dr Ferguson is of opinion that these sounds are not due to heating 
effects but to some other molecidar cause, which he does not very 
clearly define. Except in the case of iron, I see no reason as yet 
for so explaining them. It must be remembered that it is not the 
whole heating effect that is the question, but its variation in a very 
short time, say the -g-g-g-th of a second, so that there may be no 
inconsistency in explaining the ticks in Mr Ferguson’s experiments 
and the music in my own as due to the same cause. 
