PLATK CX. 



FlO. 293. The Rheochord. The diagram represents the end of the board on which tho resistance wires are 

 stretched, a, b, c. <l, c,f, g, are brass blocks which would, if it were not for the wires, be insulated. From the 

 block It a Herman silver wire (the course of which is indicated by the dotted line), after turning round an ivory pin 

 at I, returns to c. From c a similar wire of exactly the same length returns to d. From d a wire three times the 

 length returns to e; e and/are connected by a wire five times as long. From each of the blocks.* and b platinum 

 wires extend to the further end of the board, a distance of more than a metre, which are insulated at their 

 extremities. They are, however, in metallic connection by means of a slide (" travelling mercury cups ") shown in 

 the diagram. According to the distance of the slide from a and 6, which can be measured by a scale on the board, 

 the resistance between and 6 can be varied. When the slide is as far as it will go, the resistance is equal to that 

 between 6 and c, or c and d. When the slide is pushed up to u />, the total resistance of the rheochord is twenty 

 times as great as between 6 a-ad c. If plugs (not shown in the diagram) are inserted between eich block and its 

 neighbour, the resistance is nil. (See p. 347.) 



FlO. 299. Double key. 



FIG. 300. Du Bois Reyinond' key. 



