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33

The motion of the slider along the slider bar is controlled by two movements, a slide movement for rough adjustment and a lever and cam movement for fine adjustment.

Although the bridge is specially designed for platinum resistance thermometry, it is also well adapted for the accurate measurement of small resistances not exceeding 25 ohms, by the usual bridge method, to 0’0001 ohm.

The bridge is “ self-testing ” in the sense that the comparative value of the resistance coils can be tested at any time by means of the bridge itself, and their comparative errors determined. The accuracy of the bridge for thermometric work depends not upon the absolute value of the resistance coils, but upon their comparative values relative to each other and to the fundamental interval of the thermometer. These relative values are adjusted with the greatest care and, as just explained, they may be tested at any time to any required degree of accuracy.

The absolute value of the resistance unit adopted in the bridge may be taken, with good accuracy, to be 0’01 ohm, but, if required, the absolute value in terms of any specified standard may be determined exactly by measuring a suitable standard resistance with the bridge.

Fery Radiation Pyrometers. (Fig. 20.)

(1) “ Optical Pyrometry,”, by Waidner and Burgess, Bulletin of Bureau of Standards, Vol. 1, No. 2.

(2) C.R., CXXXIV., 997, 1902.

(3) “ Ann. de Chim. et de Phys.,” XXVIII., page 428, 1903.

(4) “J. de Phys.,” 8, page 701, 1904.

For temperatures from 500° C. upwards.

In the measurement of furnace-temperatures, radiation pyrometers are the only pyrometers which are entirely outside the furnace, and they are therefore especially useful for very high temperatures (above 1,200° C.) at which other forms deteriorate rapidly or are quite impossible.

The radiation pyrometers invented by M. Fery, Professor of Physics at the Ecole de Physique et de Chimie, Paris, are placed at some distance from the furnace, while no part of them is raised above the air temperature by more than 80° C.'

A 4240. n

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