138 EXPERIMENTAL PHYSIOLOGY. 



electricity passes through the former and none through the 

 latter. 



Move the slider away from A, when a certain resistance is 

 thrown into the rheochord circuit, according to the length of the 

 platinum wires thus introduced into it, and so a certain fraction 

 of the current is sent through the electrode circuit. If the 

 plug Sj be taken out more resistance is introduced, that due to 

 the German-silver wire (I b), and, therefore, a certain amount 

 of the current is made to pass through the electrode circuit. By 

 taking out plug after plug more and more resistance is thrown 

 into the rheocord circuit. The plugs are numbered, and the 

 diameter and length of the German-silver wires are so selected in 

 making the instrument, that the resistances represented by the 

 several plugs when removed, are all multiples of the resist- 

 ance in the platinum wires on which the slider moves. The 

 instrument is described here for convenience, but its use will be 

 practised later. 



(a). Connect a Daniell's cell or a small Grove with the 

 binding screws at A and B, introducing a du Bois key 

 in the circuit. To A and B attach two other wires, and 

 connect them to a du Bois key, and to the key attach 

 electrodes, thus short-circuiting the electrode circuit. 



(6). Prepare a nerve muscle preparation, lay the muscle 

 on glass, and place the nerve over the electrodes. 



(c), Put in all the plugs and push up the slider close to 

 the blocks. Open the short-circuiting key. Make the battery 

 circuit, perhaps a contraction may be obtained. Pull the 

 slider away from the blocks, and on making the current 

 contraction will occur, and perhaps also on breaking it. Take 

 out plug S lf and pull the bridge still further away, and very 

 probably there will be contraction both at make and break. 

 Proceed taking out plug after plug, and note the result. The 

 result, and explanation thereof will be referred to in Lesson 

 XLIL, 2. 



12. Pohl's Commutator. Sometimes it is desired to send a cur- 

 rent through either of two pairs of wires. This is done by means 

 of Pohl's commutator without the cross-bars (Lesson XXX., 



