554 PROCEEDINGS OF THE AMERICAN ACADEMY. 



quality cherry, 66 cm. long and 61 cm. wide, with hard-rubber bushings 

 7 mm. thick under the bars to give satisfactory clearance from the board. 

 The coils were hung from mercury cups in the copper, so as to be im- 

 mersed in a tank of oil, which was thoroughly stirred and mixed by 

 means of a simple circulating pump driven by a small electric motor. 

 In selecting this oil the aim was to obtain as pure a hydrocarbon as 

 possible, which would give no trouble from evaporation. A pure hydro- 

 carbon should introduce no difficulties on account of oxidation, and should 

 maintain a high degree of insulation indefinitely as a result of its chem- 

 ical inertness. What in the trade is known as liquid vaseline was 

 adopted — an almost colorless oil, without odor and of approximately 

 the consistency of a thin syrup. After five years no traces of deterio- 

 ration have been found. 



The general arrangement of the bridge is shown in Figure 3, where M 

 and N are the ratio coils, and P and P' the contact makers for the two 

 manganin bridge wires. The connections are shown diagrammatically 

 on Figures 1 and 2, where the numbers and letters correspond with those 

 given in Figure 3, and serve to distinguish the most important of the 

 mercury cups and coils. 



The interchange of connections as required by Figures 1 and 2 is 

 brought about by the " commutator " K, the position shown correspond- 

 ing to Figure 1, that for Figure 2 being obtained by rotating K in a 

 counter-clockwise direction through an angle of 60°, the connector from 

 5 to 7 then going from 6 to 8, while the one between 10 and 8 goes 

 from 5 to 9, etc. These heavy connectors are shown by the shaded 

 blocks in Figures 1 and 2. 



In the lower right-hand corner of Figure 3 are two reversing switches 

 for the battery and galvanometer circuits, the leads from which are 

 marked by the brackets B and G. From the galvanometer switch a 

 lead runs to the bar Z, from which, by means of a copper connector 

 carried by the rod L (of hard rubber and supported from K), connection 

 is made to Q or Q', according to the position of K. In this way when 

 the commutator K is shifted, the galvanometer lead is changed from F 

 to D or E to C automatically. 



The throw-over switch 11 allows the galvanometer lead coming from 

 Q or Q' to connect either to the ends of the platinum wire by means of 

 C and E, or to the ends of the leads D and F. Readings made iu this 

 latter case give data as stated above to determine the resistance of the 

 main leads to platinum thermometer T. (Figure 3 shows II thrown to 

 give the connections for determining lead resistance.) 



