226 



LORD KELVIN, DR. M. MACLEAN, AND MR. A. GALT, ON THE 



constant by regulating the stop-cock, C, so that the abstraction of air by the pump 

 was exactly compensated by the gain through C. 



The galvanometer measures only that part of the electricity entering the bell-jar by 

 the wire, ww, which leaves it by its metal base. This part is very nearly the whole. 

 The observed results show that it is enormously great in comparison with the very 

 small part which is carried away in the current of air to the electric filter. 



The galvanometer was shunted by a battery of LEYDEN jars, J, to give steady 

 deflections. Its sensitiveness was l/22'l of a mikro-ampere per scale division. 



83. First, we kept the potential of the needle constant throughout a set of 

 experiments made at different air-pressures, and in this way we found that the 

 current through the air to the metal of the jar became greater as the pressure of the 

 air in the bell-jar became less, down to the lowest pressure to which we went, which 

 was 40 millims. Curve 10 shows the relation between the current and the air- 

 pressure at a potential of 5000 volts. Similar curves were got for other electric 

 potentials. 



Curve 10. 



16 



15 



14 



13 



I? 



10 

 <o 



I- 



t- 



' 



\ 



ent 



Folt 



'at 



l s, 7 vsit 



100 



^oo 



300 



400 



500 



600 



700 



300 



Pressure gfrfir in Millimetres cf Jlfercurt/, 



