230 



LECTURES TO SCIENCE TEACHERS. 



spoken of. The suspension of the needle is different. The 

 needle is firmly attached at right angles to a tightly- 

 stretched platinum wire, the lower end of which is fixed, 

 and the upper mechanically connected with, though elec- 

 trically insulated from, the parts that correspond to a 

 torsion head. See Thomson's Electrostatics and Magnetism, 

 xvi., 277.] 



I will conclude my lecture for to-day by showing you an 

 electrometer, the first of a new class, and one of great 

 interest. This is Thomson's first " divided ring " electro- 

 meter, 



Fio. 3. 



In this instrument there is a broad metallic ring cut 

 into two parts, and each half ring is supported horizontally 

 on two vertical pillars of thin glass rod. One half ring is 

 kept connected with the case of the electrometer ; the other 

 can be connected by means of a proper electrode with any 

 body to be tested. From the top of the case of the electro- 

 meter hangs a fine glass fibre which carries a light 

 aluminium needle, projecting from a point a little above 

 the centre of the divided ring. The needle being counter- 

 poised projects out on one side only. It is sufficiently long 

 to project out over the divided ring. 



