CONSTANT CURRENT. 



153 



that only one surface of their platinum terminals is ex- 

 posed under it. Pull the secondary coil (S) far away from 

 the primary (P), raise the short-circuiting key (K'), make and 

 break the primary circuit by means of the key (K). At 

 first there may be no contraction, but on approximating the 

 secondary to the primary coil a single muscular contraction 

 will be obtained, first with the break shock, and on ap- 

 proaching the secondary nearer to the primary coil, also 

 with the make. The one is called a make and the other a 

 break contraction. Record the results obtained. 



2. Interrupted Current. 



(a.) Arrange the induction machine so as to cause Neef's 

 hammer to vibrate as directed in Lesson XXV, 2. On 

 applying the electrodes to the sciatic nerve or gastrocnemius 

 muscle, at once the muscle is thrown into a state of rigid 

 spasm or continuous contraction, called tetanus, this condi- 

 tion lasting as long as the nerve or muscle is stimulated, or 

 until exhaustion occurs. 



3. Constant Current Apparatus. One, two, or three Daniell's 

 cells, du Bois key, four wires and pair of electrodes, forceps, 

 and nerve-muscle preparation. 



(a.) Use two Daniell's cells. If two or more Daniell's 

 cells be used, always connect them in series i.e., the 

 positive pole of one cell with the ne- 

 gative pole of the next one. Connect 

 two wires, as in Fig. 55, to the free 

 + and - poles of the battery (B), and 

 introduce a du Bois key (K'), so as to 

 short-circuit the battery circuit. Fix 

 two shielded electrodes in the other 

 binding-screws of the du Bois key, 

 and having prepared a fresh nerve- 

 muscle preparation, lay the divided 

 sciatic nerve (N) across them, as 

 shown in Fig 55. 



(6.) Make and break the current, 

 and a single muscular contraction or 

 twitch is obtained either at making 

 or breaking, or both at making and breaking. 



Fig. 55. Scheme of con- t 

 stant current. B, bat- 

 tery ; K', short-cir- 

 cuiting key; N, nerve; 

 M, muscle. 



Notice that 



