indicate the temperature of an inaccessible spot. A coil of platinum or platinum-alloy wire is enclosed in a suitable fire-proof case and put into the furnace of which the temperature is wanted. Connecting wires, properly protected, lend from the coil to a differential voltameter, so that, by means of the current from a battery circulating in the system, the electric resistance of the coil in the furnace can be determined at any moment. Since this resistance depends on the temperature of the furnace, the temperature call be found from the resistance observed. The instrument formed the subject of the Bakerian lecture for the year 1871.
Siemens's researches on this subject, as published in the Journal of the Society of Telegraph Engineers (Vol. I., p. 123, and Vol. III., p. 297), included a set of curves graphically representing the relation between temperature and electrical resistance in the case of various metals.
The electric pyrometer, which is perhaps the most elegant and original of all William Siemens's inventions, is also the link which connects his electrical with his metallurgical researches. His invention ran in two great grooves, one based upon the science of heat, the other based upon the science of electricity; and the electric thermometer was, as it were, a delicate cross-coupling which connected both. Siemens might have been two men, if we are to judge by the work he did; and either half of the twin-career he led would of itself suffice to make an eminent reputation.
The success of his metallurgical enterprise no doubt reacted on his telegraphic business. The making and laying of the Malta to Alexandria cable gave rise to researches on the resistance and electrification of insulating materials under pressure, which formed the subject of a paper read before the British Association
