1 68 PRACTICAL PHYSIOLOGY. [XXVII. 



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

 it again touches 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 

 milled head 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 (fig. 95). 



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

 1 6) 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 pieces 60-90 

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

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

 solder thicker copper wire I inch long. 



(6.) 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 



FlO. 95. Ewald's Sledge Imluctoriuni. S. Secondary coil moved by milled head R; K. 

 Core of primary coil; A. Milled head 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., 66 Fenchurch Street, London, and costs 4, 10s.) 



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 two 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 copper 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 purposes, e.g., stimulation of the vagus, 

 these electrodes are used, ?'.., the platinum terminals are exposed only on one 

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



