1890.J 



Analogue of the Wheatstone Bridge, 



465 



A is 4 ft. x 1\ ft. x 11 ft., and B 1 and B 3 are each 3 ft. x 1 J ft. x 1| ft. 

 The ends of B x and B 2 abut against the side of A, as shown in the 

 figure ; between B x and A is a rectangular opening, a 2 , 1 in. x •J in., 

 in a cardboard diaphragm, and between B 2 and A a rectangular 

 opening, a 4 , 1 in. X 1 in., in a similar diaphragm. In the side of 

 Bjl at a x is an adjustable slit, made by cardboard shutters sliding in 

 cardboard grooves, and at a 3 in the side of B 2 , opposite to <x l5 is a 

 similar adjustable slit. The tube connecting B x and B 2 , or "galvano- 

 meter" tube, is a straight tube of glass, G, of about 1*1 inch internal 

 diameter. It can be closed at one end by a small trap-door, D, in 

 the interior of the box B 1} which can be opened and shut by a steel 

 wire, S, passing through a cork in the top of B x . The sensitiveness 

 of the apparatus depends upon the indicator employed. There are 

 many indicators that might be employed ; the one I have tried and 

 have found to work well consists of two very small parallel sewing 

 needles, stuck through a cap of elder-pith, supported on a small 

 agate compass centre; the needles carry very light mica vanes on 

 one side of the centre, counterpoised by a small quantity of platinum 

 wire. The whole is balanced on the point of the finest needle I could 

 obtain, and forms a very delicate wind vane. When first mounted, 

 the needles always took up a position of equilibrium with the points 



