IF two strips of different metals, such as silver and iron, be soldered
together at one end, and the other ends be connected with a
galvanometer, on heating the soldered junction of the metals it will be
found that a current of electricity traverses the circuit from the iron
to the silver. If other metals be used, having the same size, and the
same degree of heat be applied, the current of electricity thus
generated will give a greater or a less deflection, which will be
constant for the metals employed. The two metals generally employed are
bismuth and antimony, in bars about an inch long and an eighth of an
inch square. These are soldered together in series so as to present for
faces the ends of the bars, and these often number as many as fifty
pairs. Such a series is called a thermo-pile. This method of generating
electricity was discovered by Seebeck of Berlin in 1821, but the
thermo-pile so much in use now in heat investigations was invented by
Nobili in 1835. The strength of this current is not very great, a single
Daniell cell being equal to nine pairs of the strongest combination yet
discovered, namely, the artificial sulphuret of copper with German
silver.