1881.] Determination of the Ohm in Absolute Measure. 129 



moveable contact of the galvanometer circuit with the bridge wire, is 

 eliminated in Foster's method, but to ensure greater accuracy and 

 safety all measurements were repeated with reversed battery current. 

 The whole comparison seldom occupied more than five minutes ; and 

 the readings obtained with the battery current in different directions 

 closely agreed with each other. 



The spinnings were always taken in sets of four at the same speed, 

 and the comparison of resistance was made at the beginning and 

 end of each set. During the time of spinning the resistance was 

 found to have altered owing to a rise of temperature which always 

 took place during the time of experimentation. The corrections for 

 the change of resistance and the possible errors introduced owing to 

 the uncertainty of this correction will be described further on. 



After the resistance of the coil had been measured, it was discon- 

 nected from the balance and set into rotation with open circuit, so that 

 no current could pass. While the water supply was adjusted so as to 

 give approximately the required speed, the magnet in the centre of 

 the coil, which had been strongly disturbed during the measurement 

 of resistance, was brought to rest either by means of an outside 

 magnet or more often by means of a small coil and Le Clanche cell, 

 which was always placed in the neighbourhood of the rotating coil. 

 A key within reach of the observer served to make and break contact 

 at the proper time. When the speed had been approximately regulated 

 and the magnet was vibrating through a small arc only, its position of 

 rest was determined, while at the same time the auxiliary magneto- 

 meter placed in the adjoining room was observed. The two ends of the 

 rotating coil were now connected together, by means of a stout piece 

 of copper, the well amalgamated ends of which were pressed into cups 

 containing a little mercury, into which they tightly fitted. 



As the coil was set into rotation the magnet slowly moved towards 

 one side, and a proper use of the damping coil brought it quickly to 

 approximate rest near its new position of equilibrium. When the 

 swings extended through no more than ten or twenty divisions of the 

 scale, the coil was kept, as nearly as possible, at the proper speed, by 

 the observer at the tuning fork (Lord Rayleigh, see p. 112). Read- 

 ings of the successive elongations were taken for about two minutes, 

 and a signal given at the beginning and end of each set of readings 

 enabled the observer at the auxiliary magnetometer (Mrs. Sidgwick) to 

 note its position as well as any changes in the direction of the earth's 

 magnetic force during the time of observation. The direction of 

 rotation was now reversed, and the deflection observed in the same 

 manner ; the whole process being twice repeated, so that four sets of 

 readings were obtained. When all the observations for the given speed 

 had been completed, the position of rest of the magnet, when no 

 current passed through the coil, was again determined and compared 



