of Edinburgh, Session 1879-80. 
731 
so transmitted, and in one case I managed to do this so successfully 
that a listener was able to write down an unknown sentence spoken 
into the mechanical telephone. 
I found also that certain lengths of the fine iron wire suited 
certain voices better than others. 
Still using the same fine wire at A, I removed one by one the 
cells from the battery till only one remained, and found that this 
did not produce any marked effect on the intensity of the sound as 
heard at B. The last cell was then removed, and the circuit joined 
up, and even then a loud sound uttered at A could be faintly heard 
at B. The only, at least obvious, reason for this effect appears to 
be, that the vibrations of the iron wire across the lines of force due 
to the earth’s magnetism produces a current sufficiently strong and 
variable to work a telephone. 
The battery being again included in the circuit, a horse-shoe 
magnet with the line of its poles at right angles to the wire was 
suspended over and quite close to the wire. This caused the sounds 
uttered at A to be reproduced at B with distinctly greater intensity 
than when no magnet was present. To make sure of this a con- 
tinuous note was sounded at A, and the magnet alternately removed 
and brought up to the wire. When this was done a marked rise 
and fall in the intensity of the sound heard in the receiving telephone 
was observed. 
It is to be noticed that, in the case of the fine iron wire, the sounds 
appeared to be transmitted equally well whether the wire attached 
to the mechanical telephone was joined to the middle of, and at right 
angles to, the inserted wire, or to its end, and in the same direction as 
itself. This observation is of importance, as it does away, in great 
part at least, with the idea that the effect is produced by variation 
in the resistance of the fine wire caused merely by the bending to 
and fro at its points of attachment to the circuit wires. 
When the fine iron wire was replaced, all other things remaining 
the same, by a thick iron wire (No. 12), which had been previously 
rubbed with a magnet, the sound heard at B was very faint, although 
audible. It came out, however, very distinctly where the iron wire 
was heated by a flame to a dull red heat. 
With a thickish platinum wire inserted at A the sound produced 
at B was very faint ; but on putting in about twelve inches of fine 
