208 PHYSIOLOGY 



and if the binding screws of the two elements are connected by a wire, the 

 current flows in the wire (outer circuit) from copper to zinc, thus completing 

 the circuit. Since in the outer circuit the current flows from copper to zinc, 

 the terminal attached to the copper is called the positive pole, and that to the 

 zinc the negative pole. When the current is required to be very constant, the 

 zinc may be immersed in a saturated solution of zinc sulphate instead of dilute 

 sulphuric acid. A Daniell's cell, though very constant, gives only a small 

 current, owing to its small electromotive force and high internal resistance. 



When a stronger current is required it is best to use a storage battery. In 

 this, 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 very convenient form of battery, though not so constant as the 

 two forms just described, is the bichromate battery, with a single fluid. This 

 consists of a plate of zinc between two plates of carbon. The whole are arranged 

 so that they can be immersed in or drawn out of the fluid at pleasure. The 

 fluid used is a mixture of sulphuric acid and potassium bichromate. The wire 

 attached to the carbons is the positive pole and the current in the outer circuit 

 flows from carbon to zinc. 



Another useful type of cell is the Leclanche cell. This consists of a glass 

 jar containing 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 surrounded 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 connected 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 polarisation 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, par- 

 ticularly 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 14 

 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 Daniell's 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 completing 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 anode, and the 

 point at which the current leaves the nerve is called the cathode. The wires 

 by which the current is conducted to and from the nerve are called the elec- 

 trodes. As electrodes we generally employ two platinum wires mounted together 

 on a piece of vulcanite. 



