in the Use of Bijilar Suspensions. 441 



in potential across this differed from the electromotive force 

 of a normal Weston cell by only a few millionths of a volt. 

 The cell was one of a number constructed according to the 

 specifications of Wolff and Waters, and kept in a thermostat 

 at a temperature of 25° C. constant to '01° C* By opposing 

 these two potential differences and measuring their difference 

 on an ordinary potentiometer, with a sensitive galvanometer 

 in the circuit, it was possible to adjust the current to a con- 

 stant value. This was done by means of special rheostats 

 capable of very delicate adjustment in the main circuit. It 

 was not necessary to always adjust the current in order to 

 obtain a given reading, for if the difference on the potentio- 

 meter was read, the value of the current could always be 

 expressed in terms of that current which would produce a 

 zero difference between the E.M.F. of the cell and the drop 

 in potential across the resistance. Observations of the de- 

 flexion were taken simultaneously with a potentiometer 

 reading, which never exceeded 700 microvolts, and was 

 usually less than 100. As this variation is so small and 

 could itself be obtained to better than 2 microvolts, it was 

 always possible to apply to the deflexion a correction which 

 could not be in error by as much as *001 cm. on the scale. 

 For example, if the drop in potential was 100 microvolts 

 greater than the E.M.F. of the Weston cell, then the current 

 would have to be cut down one part in 10189 to get the 

 value which would produce the same drop in potential as 

 the Weston cell. As the circuits connecting the fixed and 

 suspended coils of the dynamometer were connected in 

 parallel (the resistance being in the suspended coils circuit), 

 and as the deflexion varies with the product of the two 

 currents, we see that the deflexion will be reduced by two 

 parts in 10189, or in our case where the readings all lie 

 between 42'3 and 42'4 cm. the correction would be '0083 cm. 

 We could thus determine this more closely than we could 

 read our scale, and were able to save considerable time and 

 trouble in regard to keeping the current absolutely constant. 

 The telescope and scale arrangements (also described by 

 Callendart) enabled the deflexion to be obtained to '002 cm., 

 but as it was necessary to take readings at definite intervals 

 during the creep, occasional uncertainties of as much as 

 •005 cm. might arise, which were due to the necessity of 

 bringing the suspension absolutely to rest before reading. 

 This was performed by means of a special damping device, 

 which made it possible to produce the deflexion and bring it 



* Wolff and Waters, Bull. Bur. Stan. vol. iv. 1907, p. 1 ; and Bronsoa 

 and Shaw, British Assoc. Report, 1009, p. 396. 

 f Loc. cit. 



