ELECTRICAL KEYS, RHEOCHORD. 



137 



slides along on the two wires. Connect the battery a key being 

 interposed in one wire with the binding screws on one end of 

 the rheocord, and to the same binding screws connect two wires 

 to the detector. Observe as the mercury cup is pulled away from 

 the binding screws, there is a greater deflection of the needle, but 

 the deflection is not in proportion to the distance of the cup. 



11. The Rheochord of du Bois-Reymond is used to vary the 

 amount of a constant current applied to a muscle or nerve. It 

 consists of a long board or box, with German-silver wire of 

 varying length and whose resistance is accurately graduated 

 stretched upon it. At one end are a series of brass blocks dis- 

 connected with each other above, but connected below by a 

 German-silver wire passing round a pin. These blocks, however, 

 may be connected directly by brass 

 plugs S x S 2 ... S 5 . From the 

 blocks 1 and 2, two platinum wires 

 pass from A to the opposite end of 

 the box (Y), where they are insu- 

 lated. Between the wires is a 

 " slider " (L), consisting of two cups 

 of mercury, which slide along the 

 the wires. 



In using the instrument, take a 

 Daniell's battery and connect its 

 wires to the binding screws at A 

 and B, and to the same screws 

 attach the wires of the electrodes 

 over which the nerve (c d) of the 

 muscle (F) is laid. We have two 

 circuits (a c d b and a A B b}, the 

 wires of the rheochord introduced 

 into the latter. 



Push up the slider with its cups 

 (L) until it touches the two brass 

 plates 1 and 2, and insert all the 

 plugs (Sj-S 5 ) in their places, thus 

 making the several blocks of brass 

 practically one block. In this posi- 

 tion the resistance offered by the 

 rheochord circuit is so small as 

 compared with that including the Fig 44. Rheochord of du Bois- 

 nerve, that practically all the Reymond. 



