296 



THE ELECTRIC TELEGRAPH. 



is connected in the same circuit as the upper branch, r, of 

 the proportion resistances r and p, by which the resistances in 



these branches may be 



r = 10,100, 1000, or 10000, 

 in the upper, and 

 ftZ p = 10, 100, or 1000, 



in the lower side. 



The upper circuit includes, also, a 

 triangular commutator, U 3 , for com- 

 pleting this circuit, with or without 

 the inclusion of a Daniell's element. 

 The triangular commutator consists 

 of three brass slabs i, 11, and 

 HI. The Daniell's cell is connected 

 between n and in, while i and n 

 are in circuit of the bridge. Be- 

 tween i ii and i in are holes 

 Fig. 140. for a contact plug, which, when 



put in the hole i IT, completes the circuit without the 

 element ; and when in i m, introduces the element into 

 the circuit. 



The point of contact, c, of the upper branch, r, with the 

 lower, p, is connected with the beam of a key, k, the back or 

 reposing contact of which is to earth ; the front or working 

 contact to one corner of the four- sided current director, Uj. 

 The ends of the galvanometer coil are not connected imme- 

 diately to the corners of the parallelogram, but are inter- 

 cepted by a current-director, u 2 , for enabling the operator to 

 observe the deflection of the needle on either side of the 

 Zero line. Three metal slabs, L, L L , and E, with holes 

 between them, for the insertion of contact plugs, form the 

 upper right-hand side of the bridge. The earth-plate is 

 connected by a wire to E ; L X is connected with the upper 

 corner of the bridge ; L with the conductor of the cable, at one 

 end, and should both ends be at the disposition of the operator, 

 he connects the other end to E, when he wishes to measure 

 the copper resistance. The purpose of these three slabs is 



