the Roberts- Austen Recordiny Pyrometer. 07 



immersed in paraffin, which could be kept in circulation by 

 means of a stirrer. Experiments on the relative resistances 

 of the high and low resistance-coils failed to show that any 

 change was produced by the passage of the extremely small 

 potentiometer current, and the use of the paraffin was, 

 therefore, not resorted to. 



The current flowing through the potentiometer is main- 

 tained by means of a Clark cell g with large electrodes, each 

 electrode being 20 sq. in. in area. The Clark cell is placed 

 in series with a resistance of 2500 ohms. Under these 

 circumstances, as has been shown by S. Skinner *, the current 

 produced is remarkably constant, and there is also this 

 advantage in the use of the Clark cell, that its E.M.F. when 

 producing a current can be easily compared with that of 

 standard Clark cells of the usual form. The use of Clark 

 cells for standards has necessitated a large temperature 

 correction ; but as the cells were placed in a thick wooden 

 box D, and their temperature only rose 2 or 3 tenths of a 

 degree during the day, the rate of change of temperature 

 was sufficiently slow to render this correction fairly legitimate. 

 The E.M.F. of the large Clark cell g is compared with that 

 of the standards /*, h, h by means of the auxiliary potentio- 

 meter p, which is connected in series with a dry cell^ and 

 resistance r (the necessary connexions can easily be traced on 

 the figure) . This potentiometer measures the small difference 

 between the E.M.F. across the terminals of the main 

 potentiometer and the E.M.F. of each of the standards 

 h, h, h. By an arrangement which is not shown in the figure, 

 the E.M.F. across the terminals of p is obtained in potentio- 

 meter units, and so the readings on p can be converted into a 

 correction to be applied to the readings in B. 



The Galvanometer. — The sensitiveness of the pyrometer 

 depends on the sensitiveness of the galvanometer and its 

 constancy of zero. The difficulty of obtaining a sufficiently 

 sensitive galvanometer was increased by the necessity for 

 introducing the resistance (20 ohms) of the potentiometer. 

 In experimenting with sensitive suspended-coil galvanometers, 

 great difficulty was experienced in finding one which would 

 return to its zero position after being deflected. This difficulty 

 was at first attributed to the viscosity of the phosphor-bronze 

 strip used to suspend the coil, but it was found that the coil 

 would vibrate when suspended by a silk fibre between the poles 

 of a permanent magnet, a fact which showed that the coil itself 

 was magnetic and that the change in zero was largely due to 

 a change in the direction of the induced magnetism. 



* Phil. Mag. Sept. 1894, p. 271 ; Proc. Phys. Soc, Feb. 1895. 

 F 2 



