Tasimeter

In 1878, Samuel Langley, Henry Draper, and other American scientists needed a highly sensitive instrument that could be used to measure minute temperature changes in heat emitted from the Sun's corona during the July 29 solar eclipse, due to occur along the Rocky Mountains.

It is firmly clamped at B, its lower end fitting into a slot in the metal plate, M, which rests upon the carbon-button.

To avoid any deflection which might arise from change in strength of battery, the tasimeter is inserted in an arm of a Wheatstone bridge.

[1] In order to ascertain the exact amount of expansion in decimals of an inch, the screw S, seen in front of the dial, is turned until the deflection previously caused by the change of temperature is reproduced.

[1] Edison declined to patent the device, saying it was only of interest to scientists, and allowed companies in London and Philadelphia to manufacture tasimeters royalty-free.