58 



INDUCTION COIL. 



[BOOK i. 



other ways. Such a means of closing and opening a circuit and so of 

 making or breaking a current is called a key. 



A key which is frequently used by physiologists goes by the name of 

 du Bois-Eeymond's key ; though undesirable in many respects it has 

 the advantage that it can be used in two different ways. It may be 

 arranged as in A, Fig. 2. In this case, when the brass bridge of K, 

 the key is put down (dotted outline in the figure), so as to form a 

 means of good conduction between the brass plates to which the wires 

 are screwed, the circuit is closed and the current passes from the posi- 

 tive pole (end of the negative copper element) to the positive electrode 

 or anode, An. through the nerve, to the negative electrode or kathode 

 Kat. and thence back to the negative pole (end of the positive zinc 



B 



FIG. 2. DIAGRAM OP Du BOIS-REYMOND KEY USED, A, FOR MAKING AND BREAKING, 

 B, FOR SHORT CIRCUITING. 



element) in the battery ; on raising the brass bridge (continuous outline 

 in the figure) the circuit is opened, the current broken^ and no current 

 passes through the electrodes. Or it may be arranged as in B. In 

 this case if the brass bridge be ' down/ the resistance offered by it is so 

 small compared with the resistance offered by the nerve between the 

 electrodes, that the whole current from the battery passes through the 

 bridge, back to the battery, and none, or only- an infinitesimal portion, 

 passes into the nerve. When on the other hand the bridge is raised, 

 and so the conduction between the two sides suspended, the current is 

 not able to pass directly from one side to the other, but can and does 

 pass along the wire through the nerve back to the battery. Hence in 

 arrangement A, ' putting down the key ' as it is called makes a current 

 in the nerve, and ' raising ' or ' opening the key ' breaks the current. In 

 arrangement B, however, putting down the key diverts the current from 

 the nerve by sending it through the bridge, and so back to the battery ; 

 the current instead of making the longer circuit through the electrodes 

 makes the shorter circuit through the key; hence -this is called 'short 

 circuiting.' When the bridge is raised the current passes through the 



