262 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



Fig 278. 



Fig 279. 



If two pith balls be suspended by 

 two silk threads, and excited either by 

 glass or by resin, on removing the ex- 

 citing material, they 

 will no longer fall 

 into the vertical po- 

 sition, but repel each 

 other in the manner 

 shown in Fig. 279, 

 the balls acquiring a 

 property relative to 

 each other similar to 

 that which the glass 

 and single ball ex- 

 hibited after contact 

 in the preceding ex- 

 periment. 



Price, 50 cts. 



Fig. 280. 



le Balance Electro- 

 meter. {Fig. 280.) This 

 is an instrument of great 

 delicacy. It consists of a 

 fine beam of metal, sus- 

 pended on a pivot affixed 

 to a glass rod in the cen- 

 tre. It is accurately ba- 

 lanced, and has at each 

 end a pith ball. The glass 

 rod must be very dry when 

 the instrument is to be 

 used ; an excited stick of 



sealing-wax or glass held towards one end will occasion it 

 to move round its centre, and thus indicate plainly the ex- 

 citement of the wax, or glass. Price, $1.00. 



The Quadrant Electrometer. This instrument (Fig. 281, 

 next page) consists of an upright stem of wood, furnished at 

 the lower end with a brass ferule and pin, by which it may 

 be inserted in the conductor. To the upper part of the stem 

 is affixed a graduated semicircle of box wood, or ivory, at 

 the middle of which is supported on an axis, an index, which 

 consists of a very slender ivory rod, and reaches from its 



