Electrical Thermometry

In cases where it is desired to obtain greater accuracy without abandoning the quickness of reading which is the principal advan tage of the deflection method, it is possible to combine the two methods by balancing part of the potential difference by means of a potentiometer and using the galvanometer for the small changes only. In cases where the greatest accuracy is required, a very sensitive galvanometer should be used, and the whole of the potential difference should be balanced as nearly as possible, leaving very little to depend on the deflection of the galvanometer. The degree of sensitiveness and accuracy obtainable depends primarily on the delicacy of the galvanometer, on the power available, and on the steadiness of the conditions of experiment. For thermometry of precision the resistance thermometer pos sesses three very great advantages over the thermocouple : (I) The power available, owing to the use of a battery, is much greater; (2) it is possible completely to eliminate the errors due to acci dental thermal effects by reversing the battery; (3) the Wheat stone bridge method can be employed in place of the potenti ometer, so that the constancy of the battery is immaterial, and it is not necessary to use a standard cell. Another disadvantage of the potentiometer method as applied to resistance thermome ters, is that it is unsuited for differential measurements, which are so commonly required in calorimetric work.

Compensated Bridge Method and Apparatus.

The type of Wheatstone bridge illustrated in figure 8, page 123, though one of the oldest, is still in use for work of precision, and illus trates most of the essential points to be aimed at in work of this kind. The box measures only 15"X 6"X 3", and was designed primarily with a view to cheapness and portability at a time when apparatus for research was scarce and had of ten to be moved from one laboratory to another. The figure shows a plan of the ebonite lid, on the top of which the connections are marked in dotted lines. B1, B2 are the battery terminals, which are connected through a reversing key and a rheostat of about 200 ohms to a 2-volt storage cell. are the galvanometer terminals, con nected to a moving-coil galvanometer of about Io ohms resist ance, which is critically damped by a suitable shunt to save time in reading the deflections. The four leads of each pyrometer, several of which may be read in succession, are permanently con nected to a 4-point plug consisting of an ebonite block with amalgamated copper rods, Pi, P2 for the pyrometer leads from the platinum coil, and C1, for the compensator leads, fitting the corresponding mercury cups in the bridge. It is a matter of a few seconds only to change pyrometers, and there is very little risk of error from bad contacts. The least defect of insulation of the leads can be detected at any moment by shifting the pyrometer plug so that the points Pi, are inserted in the cups CI, leav ing C1, disconnected. This test is apt to be neglected unless it is made very easy. It puts nearly the whole voltage of the battery on to the galvanometer through the defective insulation. The cups are directly connected to the battery terminals as indi cated, and also to a pair of equal resistances of about 6 ohms each forming the ratio arms of the bridge, the middle point be tween the two being connected to the galvanometer terminal G2.

These ratio coils are separately wound with bare wire on mica plates to secure perfect insulation, and are very carefully annealed and tested for equality of temperature coefficient. They are fixed symmetrically side by side so that both are always at nearly the same temperature. The cup is connected to one end of a series of 9 resistance coils, A, B, C, D, etc., the other end of which is connected to the left side of the bridge-wire LR. The cup is similarly connected to one end of a series of compensating coils, the other end of which is connected through the millimetre scale SK to the right side R of the bridge-wire LR. The double wire MN stretched parallel to LR is connected to and to any point of LR through the sliding contact J, which carries a vernier V, reading to o•oi mm. on the scale SK.

Temperature Compensation of Resistance Coils.—In order to obtain accurate measurements of changes of resistance of the thermometers, it is necessary, either that the box coils should not change, or else that the readings should be corrected for any varia tion of temperature of the box. This can be done by observing the temperature of the box and applying a correction if the tem perature coefficient of each individual coil is known with sufficient accuracy, and if they are all at the same temperature immersed in a well-stirred oil bath. It is also possible to regulate the tem perature of the oil bath with a thermostat. But these methods are very troublesome, and would be quite unsuitable for a portable apparatus. A method of equal, if not superior, accuracy is to com pensate each individual resistance coil with a compensating coil, as shown in the figure, connected on the opposite side of the bridge-wire, both coils being cut in or out of circuit simultaneously by a 4-point mercury plug. The effective resistance of the pair is the difference between them, which will remain constant if their values are so adjusted that the increment of each per I° is the same. The resistance coils are made of platinum-silver alloy, which has a temperature coefficient about 14 times smaller than that of platinum The wire is wound on mica and enclosed in a glass tube like a platinum thermometer. The compensator is a loop of platinum wire of the same length as the platinum-silver coil. but has a resistance 14 times smaller. The two are tested together in the glass tube at temperatures between re and 3o°, and are adjusted until their difference has the desired magnitude and shows no detectable change over this range of temperature. The glass tube and leading wires are then cut to a suitable length, and the compensated pair is connected in its proper place in the box. The platinum compensating coils may be regarded as peculiarly delicate and appropriate thermometers for measuring the mean temperature of each individual coil, which would other wise be difficult. But they go a step further, and apply the neces sary correction automatically, without any waste of time cr attention on the part of the observer.