INDUCTION MACHINE ELECTRODES. 141 



6. For Non-polarisable Electrodes see Lesson XXXVIII., 2. 



7. Polarisation of Electrodes Apparatus. Pair of ordinary 

 electrodes, two wires, du Bois key, spring key, Daniell's cell, 

 frog, and instruments. 



Fig. 47. Du Bois-Eeymond's Platinum Electrodes. The nerve is placed 

 over the two pieces of platinum, P, which rest on glass ; B, universal 

 joint; V, support. 



(a.) Pith a frog (Lesson XX., 1), lay it belly downwards 

 on a frog plate, and expose one sciatic nerve. 



(b.) Clamp the du Bois key to the table, place the elec- 

 trodes under the sciatic nerve, and connect their other ends 

 each with the outer binding screw of the brass plates in 

 the du Bois key. Close the key, and observe that no con- 

 traction of the leg muscles occurs. 



(c.) By two wires connect a Daniell's cell with the du Bois 

 key, introducing a spring key in the circuit. Open the key 

 to allow the constant current to pass through the nerve for 

 three minutes or thereby, and observe that there is no con- 

 traction as long as the constant current is passing. Close 

 the key, i.e., short-circuit the battery, and at once a contrac- 

 tion occurs. Remove the battery, close and open the key. 



