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MODERN GASWORKS PRACTICE
tw • The principle of the resistance thermometer may be simply explained by the diagram, Fig. 133. The coil of platinum wire is shown at A, and this is connected with. a Wheatstone’s bridge by means of platinum wires from A to B, and of copper wires from B to C. In order to eliminate errors introduced by these
leads a set of compensation leads is placed beside the primary leads. The resistance of the platinum coil is, therefore, read direct on the Wheat-stone’s bridge, and the temperature may be taken from t li e temperature-resistancé curve for platinum. In the modern form of instrument a special indicator is employed from which the temperature
Fig. 133.—Diagram of Wheatstone’s Bbidge. can read direct. A complets resistance pyrometer, suitable for furnace use, is shown in section in Fig. 134. With this instrument four-way copper leads are connected to one side of the Wheatstone bridge and the other two to the other side.
The leads, therefore, counter-balance each other, and alter-
ations in resistance due to changes in temperature or length. are eliminated.
CALLENDAR RECORD-ING PYROMETER
This instrument (Fig. 135) is based on the Wheatstone bridge principle, but instead of the bridge having to be balanced by hånd, this is done automatically by the recorder mechanism. The pen arm is attached to the contact which slides to and fro on the bridge wire. The movement of the pen is governed by a clock,
Fig. 134.—Complets Resistance Pyrometer.
which, in tum, is controlled by two electro-magnets. The galvanometer pointer carries two insulated wires which can make contact on one side or the other of a
platinum-rimmed contact wheel. When the pen is registering the correct temperature these wires do not touch the wheel. Any change in the temperature of the thermo-