XXVI.] 



ELECTRICAL KEYS UHEOCHORD. 



I6 5 



of Oxford. It consists of a German-silver wire about 20 ohms 

 resistance, wound round ebonite pegs fixed at equal distances at 

 the opposite ends of a wooden board. The board is divided into 

 oblongs, so that each division represents y^ part of the whole 

 length of the wire, which ends in two block terminals, A, B, each 

 provided with two binding screws. One of the terminals of the 

 electrodes is attached to one terminal of the wire (A), and the other 

 to the movable block S, which represents a slider, and which can 

 be applied to any part of the wire, at any distance from A. Owing 

 to the great resistance of the nerve as compared with that of the 

 wire, the current through the nerve or muscle is in proportion 

 to the length of wire between the slider S and the block. 



(a.) Connect a DanielPs cell as in lig. 92 with the two block 

 terminals (A, B) interposing a spring-key (K). Of the electrode 

 wires one is connected to A, an I the other to the slider S. 



FIG. 92. Simple Rlieochord as used in Oxford. FIG. 93. Thomson's Reverser. 



B. Battery ; K. Spring-Key ; A. . Terminals 

 of Rheochord Wire ; S. Slider ; X. Nerve. 



Expose the sciatic nerve of a frog, and place the electrodes 

 under it, or make a nerve-muscle preparation and stimulate the 

 nerve. Place the slider close to A, there is no response either at 

 make or break. Place the slider at different distances from A, 

 and note when contraction occurs at make. 



13. Pohl's Commutator. Sometimes it is desired to send a current 

 through either of two pairs of wires. This is done by means of Pohl's 

 commutator without the cross-bars (Lesson XXXIII., fig. 112). At other 

 times it is desired to reverse the direction of a current. This is done by 

 Pohl's commutator with cross-bars. 



