554 Proceedings of the Royal Society 
then became very faint. I next slipped the wire coil entirely off 
the magnet, and simply held it touching the pole side-ways. The 
sounds were still quite audible, and continued to be so when the 
coil was removed an inch or more away from the magnet. This led 
me to try the effect of dispensing altogether with the steel magnet, 
and merely holding the helix of insulated wire opposite the centre 
of the vibrating plate. With an iron disc a very faint sound was 
heard, but it became more distinct when a disc of copper was used. 
From this it would seem, that the vibrations producing the sound 
are caused by the attraction between the currents in the helix 
and the induced currents in the copper disc. 
Still using the same transmitting instrument, I next employed a 
receiver, in which a soft iron core, carrying the insulated wire, was 
rigidly attached to the vibrating plate. This was accomplished by 
rivetting the head end of a screw-nail into its centre, and winding 
the wire round it. The pole of a steel magnet was then fixed, just 
clear of actual contact, opposite the head of the nail, and, with this 
arrangement, sounds were distinctly audible, though not so loud as 
when the wire coil surrounded the magnet. 
With the view of rendering the telephone an instrument for 
detecting the existence of very feeble currents, I constructed a pair 
with magnets similar to those already described, but with ferrotype 
discs. These were joined together by a strong semicircular spring 
in such a way that they could be put on, after the manner of 
spectacles, and stick close to the sides of the head, inclosing both 
ears. India-rubber rings were provided for the double purpose of 
excluding all extraneous sound, aud avoiding any disagreeable pres- 
sure on the ear. 
With this arrangement, I proceeded to test for the existence of 
thermo-electric currents. A copper and iron junction was inserted 
in the circuit of the telephone, and attached to a spring in such a 
way that it could be made to vibrate rapidly out and into a gas 
flame. A distinct grating noise was heard in the telephone. A 
very peculiar rasping sound was produced when the ends of a copper 
and iron wire were rubbed together while hot, and also when one 
of the wires was attached to a file, and the hot end of the other 
drawn rapidly along it. 
