i6S 



PRACTICAL PHYSIOLOGY. 



[XXVIL 



The magnetic circuit now being broken, the pendulum swings back until 

 it again touclies the contact D, when it is repelled again, and so on. 



According to the position which is given to the spring by means of the 

 milleil heail A, the amplitude and speed of the interrupter swings can be 

 varied between the limits of i and 200 per second. 



Z, Z are the battery terminals ; P and S the terminals for primary and 

 secondary current (tig. 95). 



7. Hand-Electrodes (fig. 97). — (a. ) Take a piece of double or twin wire (No. 

 16) enclosed in gutta-percha (that used for electric bells), about 6-7 cm. long 

 (2.^-3 inches). Remove the gutta-percha from the ends. By means of a file 

 taper one pair of ends to blunt points, to the other ends solder ])ieces 60-90 

 cm. long ( 2-3 feet) of thin copper wire. Coil the thin Avires round a glass or 

 wooden rod to make them into a spiral, and to their free unattached ends 

 solder thicker coj.per wire i inch long. 



(b.) Take two pieces of flexible gutta-percha coated wire (No. 20) 60 cm. 

 long, and two pieces of thick glass tubing 8 cm. long, having a bore 

 sufficient to admit the wire. Push a wire through each tube, and allow 



Fia. 95.— Ewald's Sletige Iiidncturiuni. S". Spcoiulary coil moved by milled head jR; K. 

 Core of primary coil; A. Milled h"ad to alter position of stop B ; C. Magnet; Z, Z. 

 Battery terminals; P and S. Those for primary and secondary current. (It is made 

 by A. Hurst and Co., C6 Feuchurch Street, London, and costs £4, lOs.) 



the end of the wire to project 2 cm. beyond the tube ; scrape the gutta- 

 percha off the free ends of both wires. Fix the wires in the glass tubes 

 with sealing-wax, and with a well-waxed thread bind the two tubes together. 

 Or use tAV'o pieces of No. 20 gutta-percha coated wire, each 10 cm. in 

 length, fix them in glass tubes, as shown in the figure, by means of gutta- 

 percha cement. To the ends of the copper wires solder thin silk-covered wires, 

 and to the free ends of the latter solder a short length (2 cm. ) of thick un- 

 coated copjjcr wire. A very handy holder is made by thrusting two fine 

 insulated wires (No. 36) through the bone handle of a crotchet-needle. 



8. Shielded Electrodes.— For some ])urposes, e.g., stimulation of the vagus, 

 these electrodes are used, >.<•., the ])latinum terminals are exposed only on one 

 side, the other being sunk in a uiece of vulcanite (figs. 197, 226). A pair 



