EXCITATION OF MUSCLE 187 



when charged, the two elements are lead and lead oxide, Pb0 2 . It has the advantage 

 that it may be used over and over again, being recharged through a resistance from the 

 electrical mains when it has run down. 



Another useful type of cell is the Leclanch^ cell. This consists of a glass jar con- 

 taining a solution of sal ammoniac. Into this dips an amalgamated rod of zinc, which 

 is the positive plate. A piece of gas carbon forms the negative plate. This is sur- 

 rounded by peroxide of manganese (Mn0 2 ) which is kept in contact with the surface of 

 the carbon by being placed in a porous pot. In some forms of Leclanche the manganese 

 and carbon are ground up together and pressed into a cylinder which surrounds the 

 zinc rod. When the cell is on open circuit that is, when the terminals are not con- 

 nected and no current is passing very little action takes place ; but when the circuit 

 is closed and the current passes, the zinc dissolves in the sal ammoniac, forming a double 

 chloride of zinc and ammonia, while ammonia gas and hydrogen are liberated at the 

 carbon pole. The nascent hydrogen reduces the peroxide of manganese and so polarisa- 

 tion is prevented. On account of its great solubility in water the ammonia has no 

 polarising action. The Leclanche is a convenient form of cell, as when once set up it 

 requires a minimum of attention. If it is worked through a considerable resistance, 

 it will keep in order for some time, particularly if the work is intermittent ; but if it is 

 used with a small resistance in circuit it polarises very rapidly. The E.M.F. of one 

 Leclanche cell is 1'4 volt in the external circuit. The positive current is conventionally 

 said to run from the zinc to the carbon in the cell, and from the carbon to the zinc 

 in the circuit outside. The wire attached to the carbon is the positive pole, that to the 

 zinc the negative pole. Dry cells are usually Leclanche cells, in which the solution of 

 sal ammoniac is prevented from spilling by absorption with sawdust or plaster of Paris. 

 The E.M.F. is the same as the Leclanche, but they polarise much more readily. 



If the poles of a DanielFs cell be connected by wires with a nerve or muscle of a 

 nerve-muscle preparation (as in Fig. 43), the current will flow from copper to the nerve 

 at A, and along the nerve from A to K. 

 At K the current will leave the nerve to 

 flow to the zinc of the battery, so com- 

 pleting the circuit. The point at which 

 the current enters the nerve (i.e. the 

 point of the nerve connected with the 

 positive pole of the battery) is called the g^g""" \-'''''' : \\^ K\ -*~ [A jy 



anode, and the point at which the current ^J^^^^^' Ifathode. Anode. 



leaves the nerve is called the cathode. The F 43 



wires by which the current is conducted 



to and from the nerve are called 'the electrodes. As electrodes we generaDy employ 

 two platinum wires mounted together on a piece of vulcanite. 



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. 



Du Bois Reymond's key consists of two pieces of brass, each of which has two bind- 

 ing 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 shows the way in which the key is arranged for short-circuit- 

 ing. 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 



