the Metals of the Alkalies and Alkaline Earths. 83 



apparatus constructed by Prof. KirchhoflF. The principle of Mr. 

 Wheatstone's method is as follows : — When four wires form the 

 four sides of a quadrilateral figure, with one of the diagonals of 

 which a battery is connected, and with the other a galvanometer, 

 if no current be indicated by the latter, the resistances of the four 

 sides are proportional. Suppose, then, the first of these wires to 

 be of the metal whose resistance is sought ; the second, a che- 

 mically pure silver wire ; and the third and fourth, parts of a 

 copper wire stretched on a strip of wood, and separated from 

 each other by a block of lead with a shield of copper on one side, 

 which moves along the wood, keeping the edge of the copper in 

 contact with the wire, and that a wire of the same metal connects 

 this shield with the galvanometer. Then if no current passes 

 through the latter, we have the ratio of the electric resistances 

 of the wire under observation, and the normal silver wire iden- 

 tical with that of the parts of the copper wire divided by the 

 shield, which is easily read off by means of a scale affixed to the 

 wood. 



The arrangement of the apparatus is shown in fig. 5. The 

 strip of wood, A, about 4 inches wide and 1 inch thick, has two 

 binding screws, a and «', at its middle point ; these hold the two 

 ends of a copper wire about 1 millim. thick and nearly twice the 

 length of the wood, on the surface of which it is stretched, pass- 

 ing round the pegs s and s' at the extremities; on the wood is 

 fixed a scale, by means of which the 'length of the two parts of 

 the wire divided by the shield may be read off; the shield, h, is 

 connected with the galvanometer, g, by a spiral wire. At the 

 point c in the copper wire near the binding screw a, a wire is sol- 

 dered on, of which one end dips in the quicksilver cup g, and 

 the other is connected with one pole of the battery K ; a similar 

 wire is soldered on at the point c' near the screw «', the one end 

 dipping in the cup g', and the other passing to the second pole 

 of the battery : in the same small block of wood in which are the 

 cups g and g', are two others, e and /, forming with the others 

 the four corners of a square. With / one end of the normal silver 

 wire is connected ; this wire, wound round a piece of wood, is 

 immersed in the cylmder N tilled with rock-oil; both of its ends 

 are soldered to tliick copper wires, the one going to y and the 

 other to h. Two copper wires connect the cups e and h with the 

 cups i and k, in which the wire-holders are fastened. From h 

 another wire goes to the galvanometer. Above the block of 

 wood containing the cups efgg' is another small block, through 

 which arc passed two wires twice bent at right angles, so that the 

 four ends lit in the cups below. With this arrangement we can 

 connect either fg and eg' together, or eg and fg. At each of 

 these positions of the commutator a quadrilateral figure of the 



GJi 



