GALVANIC CELLS. 185 



sulph-ion from the copper sulphate, and copper is deposited upon the 

 copper plate. The action of this cell remains constant as long as 

 there is copper sulphate present to be decomposed. A store of the 

 salt is kept in the solution to make up for loss. Voltage 1*9, 

 internal resistance 0'5 ohm approximately. 



Leclanche cell. In this the acid is replaced by sal ammoniac, 

 the solution of the zinc yields chloride of zinc and ammonia.* 

 Ammonia and hydrogen gas are liberated at the carbon. Depolari- 

 sation is effected by means of manganese dioxide, which is packed 

 round the carbon plate mixed with granulated carbon. Mn0 2 yields 

 oxygen slowly, this combines with the hydrogen to form water. The 

 hydrogen collects faster than it is removed, and the cell polarises. 

 If the circuit be open for a short time, the cell depolarises ; it is 

 therefore most useful for open circuit work, i.e., work in which the 

 circuit is closed for a brief time only. Voltage 1'5, internal resistance 

 about 1 ohm. 



This battery is mostly employed for actuating induction machines 

 and time markers, and works well provided the period of close 

 circuit is shorter than that of open circuit. Most "dry" cells are 

 modified Leclanche cells. 



Other cells used less frequently in physiological work. 



The GPOVC cell. The zinc is placed in dilute sulphuric acid in the outer 

 vessel, and platinum in a porous vessel which contains strong nitric acid (depolariser). 

 The hydrogen liberated is oxidised to water, and nitrogen trioxide fumes are given 

 off (the great disadvantage). Voltage 1'9, internal resistance may be as low as 

 ()! ohm. 



The bichromate OP Gpenet cell. A bottle-shaped receptacle, with a 

 neck as long as the zinc plate. To the stopper two carbons and a central zinc plate 

 are attached. The sensitising fluid is dilute sulphuric acid, as in the other batteries, 

 whilst for depolarising 8 p-c. bichromate of potassium is added. A small quantity 

 of mercurous sulphate in the solution helps to maintain the amalgamation. Both 

 elements dip into the same fluid. From this the zinc can be withdrawn when the 

 battery is not in use. The voltage may attain to over 2 volts, but soon drops and 

 consequently the cell is only of use for short periods. The internal resistance is 

 low. Disadvantage, frequent recharging is necessary, and creeping of the fluid 

 destroys the connection of the carbon with its terminals. 



Connections OP leads. No. 18 cotton-covered wire. The ends 

 must be bared of their covering and cleaned by scraping before bein^ 

 attached to the binding screws in order to secure good contact. 



Never omit to examine the wires for fractures, especially if they 

 are wound into spirals. 



Keys. These are inserted into the circuit wherever it is desirc-d 

 to control the passage of a current. 



