228 LECTURES TO SCIENCE TEACHERS. 



vertically and nearly reaching the bottom of the jar, and a 

 fine platinum wire with a little weight of platinum attached 

 to it hangs down from the end of the stiff wire and dips 

 into the sulphuric acid at the bottom. Thus the needle is 

 kept connected with the inside coating of the Leyden jar. 



Bather less than half-way from the bottom of the jar 

 there is a metallic ring, cemented to the inside for the pur- 

 pose of supporting two repulsion plates. This ring and the 

 repulsion plates are connected with the interior tinfoil 

 coating. When, therefore, the jar is charged with electricity, 

 repulsion takes place between the needle, also connected 

 with the inside coating, and the repulsion plates, and the 

 needle is driven up against two stops, connected with the 

 plates, which limit its motion. 



The ends of the needle and the repulsion plates are sur- 

 rounded by a cage of fine brass wire, which is stretched on 

 a brass framework. The framework is supported from the 

 metallic case of the electrometer by two glass pillars. It 

 is insulated from the case and from the needle and repulsion 

 plates ; and it has an electrode projecting outward through 

 a hole in the outer case, by means of which the cage can 

 be connected to any body that is to be tested. 



Let me now explain the use of the instrument. The 

 Leyden jar is charged by means of an electrode provided 

 for the purpose, and, as I have said, the needle is repelled 

 from the repulsion plates against the stops. The electrode 

 of the cage is connected with the earth, or more usually 

 with the brass case of the instrument, which is connected 

 with the earth. The torsion head is now turned in such a 

 way as to oppose the torsion of the wire to the electric 

 repulsion, and the needle is forced away from the stops. 

 The observer, looking down through the plate glass cover, 

 now brings the needle, by means of two marks, one on the 

 glass cover and the other on a ring below, into a " marked 

 position," and then reads off the number of degrees of 

 torsion on the scale over which the torsion index moves. 

 This reading is commonly called the " earth reading," as 

 the tested conductor the cage is in connection with the 

 earth during the observation. The number of degrees of 

 torsion required to bring the needle into the marked position 

 depends upon the electrification of the jar ; and it can readily 

 be shown that the potential to which the jar is charged, 



