Professor Tyndall has shown that all variations of temperature, in
metals at least, excite electricity. When the wires of a galvanometer
are brought in contact with the two ends of a heated poker, the prompt
deflection of the galvanometer-needle indicates that a current of
electricity has been sent through the instrument. Even the two ends of
a spoon, one of which has been dipped in hot water, serve to develop an
electric current; and in cutting a hot beefsteak with a steel knife and
a silver fork there is an excitement of electricity. The mere heat of
the finger is sufficient to cause the deflection of the galvanometer;
and when ice is applied to the part that has been previously warmed,
the galvanometer-needle is deflected in the contrary direction. A small
instrument invented by Melloni is so extremely sensitive of the action
of heat, that electricity is excited when the hand is held six inches
from it.