EXCITATION OF MUSCLE 



209 



For the purpose of making or breaking the current at will, various 

 forms of keys are employed. The ordinary make and break key consists of 

 a hinged wire dipping into a mercury cup. When the wire is depressed so 

 that it dips into the mercury, the circuit is complete. On raising the wire by 

 means of the handle, the circuit is broken. 



Muscle. 



Jfathode. 

 FIG. 43. 



Anode. 



Ntrve. 



Du Bois Raymond's key consists of two pieces of brass, each of which has 

 two binding screws for the attachment of wires. These are connected by a 

 third piece, or bridge, which is jointed to one of the two side bits, so that it 

 may be raised or lowered at pleasure (v. Fig. 44). It may be used either as 

 a simple make-and-break key, or, as is more usual, as a short-circuiting key. 

 In the first case one brass bank is attached to one terminal, the other to the 

 other terminal. If the bridge be now lowered, the connection is made and 

 the current passes. If the bridge be raised, the current is broken. Fig. 44 A 

 and B show the way in which the key is arranged for short-circuiting. It 

 will be seen that four wires are attached to the key ; two going to the 

 battery, and two we may suppose going to a nerve. When the bridge is 

 down, as in Fig. 44 A, the current from the cell on coming to the key has a 

 choice of two routes. It may either go through the brass bridge, or through 

 the other wires and nerve. The resistance of the nerve however is about 100,000 

 ohms, whereas that of the bridge is not the thousandth part of an ohm. When 

 a current divides, the amount of current that goes along any branch is inversely 

 proportional to the resistance. Here the resistance in the nerve-circuit is 



FIG. 44. Du Bois key, closed. Du Bois key, open. 



practically infinite compared with that in the brass bridge, and so all the current 

 goes through the bridge and none through the nerve. We say then that the 

 current is short-circuited. 



It is often necessary to reverse the direction of a current through a nerve - 

 muscle preparation or a galvanometer in the course of an experiment. For 

 this purpose Pohl's reverser may be used. It consists of a slab of ebonite or 

 paraffin or other insulating material, in which are six small holes filled with 

 mercury. A binding screw is in connection with the mercury in each of these 

 holes. Two cross- wires (not in contact with one another) join two sets of pools 

 together, as shown in Fig. 45. A cradle consisting of two wires joined by an 



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