METHODS AND APPARATUS. 



15 



circuited by knife switches, while V 3 and V 4 are made up from two dial switches. 

 The wires joining the main circuit to the shunted potentiometer are connected 

 to the terminals of the shunt rather than to the terminals of the potentiometer, 

 since the resistance of the latter is very large as compared with the former. 



Potentiometer. The potentiometer used is a combined potentiometer and 

 Wheatstone bridge, made by Wolff. 1 In this type of instrument, the resist- 



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Fig. 2. Complete wiring diagram of apparatus. Current from the battery B flows through the resist- 

 ances R, Vi, Vs, V3, and V4, and then divides, part flowing through the shunt S and part 

 through the potentiometer P, the total current finally returning to B through the resist- 

 ance M. The potentiometer, which is composed of 5 variable resistances is joined to the 

 switching arrangement, of which G2 connects the galvanometer G in circuit, T replaces 

 the thermal-junction system when testing, and 1, 2, 3, 4, etc., are connected to the thermal- 

 junctions at the body and the constant-temperature oven. The constantan wires are 

 indicated by the heavy lines. N is the standard cell, protected by the high resistance 

 O, and connected to the circuit through the double-contact key Gi. The switch K is for 

 convenience in bringing the galvanometer to rest. 



'Wolff, Zeitsch. f. Instrumentenkunde, 1903, Oktober. 



