6 OR

bridge methods. It is direct reading with ranges - 200° to O°, 0° to 200°, 200° to 400°, &c., up to 1,200°, on the one instrument.

A deflection of 90 angular degrees is produced by 200° of temperature with a platinum thermometer of 1 ohm fundamental interval. The accuracy can be easily checked by immersing the thermometer in ice and steam. At the National Physical Laboratory the error up the 1,200° has been found not to exceed 1000: The instrument can be calibrated in platinum degrees for use with any make of platinum thermometers.

Platinum Resistance Thermometers.

The platinum is in the form of flat strip to minimise the effect of heating by the measuring- current. The mounting depends on the temperature to be measured. They are made of 1 ohm "fundamental interval,” that is the change in resistance from 0° to 100° C. is 1 ohm. The total resistance at 0° is made 3 ohms by the addition of a, small resistance without temperature co-efficient. In this way all thermometers are interchangeable for use with the Direct Reading Indicator or any other piece of apparatus. Compensating leads are provided in the usual way.

Drysdale Standard Resistance Coil, 1 ohm. (Fig. 13.)

FIG. 13.

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