XXVII.] INDUCTION MACHINE — ELECTRODES. 



169 



of shielded electrodes is easily made by fixing the ends of two fine wires — 

 arranged parallel to each other and about one-eighth of an inch apart — in a 

 thin layer of gutta-percha cement. A little of the cement is scraped oflf to 

 expose a small piece of both wires. 



9. Du Bois-Reymond Electrodes (fig. 98).— The two wires end in triangu- 

 lar pieces of platinum (P) which rest on a glass plate. The whole is sup- 

 ported on a stand (V), and can be moved in any direction by the universal 

 Joint (B). 



FlQ. 96.— Inductorium with Secondary 

 Coil Moving in a Vertical Slot. 



TlQ. 97. — Hand Elec- 

 trodes, such as a Stu- 

 dent is required to 

 make for himself. 



10. Polarisation of Electrodes.— When a constant current is 

 led through a nerve for some time it causes electrolysis where the 

 metallic wires come into contact with the liquids of the nerve. 

 The excitability of the nerve is altered by the secondary electro- 

 motive changes thus produced, so that the nerve is thereby excited, 

 and the muscle is thrown into contraction. Apparatus. — Elec- 

 trodes (fig. 97), two wires, Du Bois key, Daniell's cell, frog. 



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

 a frog-plate, and expose one sciatic nerve. 



