the Point of an Electrified Needle. 



545 



en paraffin blocks along the floor into the adjoining room to 

 one terminal of the electrometer Q, the other terminal of 

 which was connected to earth. The whole of the wire and 

 the electrometer were enclosed in a metallic earth-connected 



Fio-. 3. 



case J of which the needle A itself formed a part, so that the 

 only part of the system which was exposed to external elec- 

 trical influences was the surface of the proof-plane itself. This 

 casing was found very necessary. The deflexions of the 

 -electrometer were read through a window L in the case pro- 

 tected with wire-netting. The proof-plane together with the 

 attached electrometer quadrants were connected to earth 

 through the key K, which was normally closed but which 

 could be temporarily opened by means of an electromagnet 

 controlled from the key M placed upon the discharging- 

 key H. One terminal of the magnet was connected to earth 

 through the case J, whilst the other was connected through a 

 dry-cell X to M ; M when closed made contact with earth, 

 thus completing the circuit and opening K. The permanent 

 connexion of the electromagnetic circuit to earth was found 

 important. 



The method of taking the readings was as follows : — The 

 screen F was charged positively by means of the Wimshurst 

 rather above the required potential (considerably over 6000 

 volts) and was then allowed to fall to that potential by 

 leakage accelerated, if necessary, by touching the wire Gr 

 with a piece of wood. A charge was induced upon the end 



Phil Mag. S. 6. Vol. 13. No. 76. April 1. 07. 2 P 



