19 

 BATTERIES 



In portable electrical and electronic apparatus for field use the most generally 

 practicable sources of electric power are the hand, wind, or engine-driven 

 generator and the storage battery. Unless the power supply requirements 

 are particularly heavy the battery is usually the most convenient. In the 

 laboratory there are certain applications for which battery power is more 

 suitable than that derived from the supply mains; these include the supply 

 of power to rather ad hoc apparatus whose expectation of life is short, and 

 for which it is not worth building a special power pack ; the supply of HT 

 power to the early stages of amphfiers where absence of ripple is essential 

 and where the current demand is so small that replacement costs are unimpor- 

 tant; the supply of extra HT power to photomultiplier cells, for the same 

 reason; the supply of LT (cathode heater) power to the early stages of ampli- 

 fiers, where it is cheaper to have a simple charger to replenish accumu- 

 lators between experiments than an elaborate device for making smooth and 

 stable d.c. directly from the mains; where a floating supply is needed having 

 a low capacitance to earth (d.c. amplifier balancing battery, Figure 12.36 

 and d.c. pentode cathode follower. Figure 11.16); last but not least, as a 

 source of emergency power in experimental runs which would be ruined by a 

 mains supply failure at a crucial moment. 

 We now discuss some characteristics of the commoner batteries. 



PRIMARY BATTERIES 



Dry Leclanche — the ordinary 'dry batteries' 



These are characterized by ready availability, clean-ness, and a wide range 

 of shapes, sizes and voltages, but their electrical performances do not seem 

 to be widely known. The figures for current outputs contained in the tables 

 below were derived from information supplied to the writer by the courtesy 

 of the Ever Ready Company (Great Britain) Ltd. 



Dry Leclanche cells are commonly made in two forms, the traditional 

 cylindrical type in which the electrolyte and depolarizing ingredients are 

 contained in a zinc cup — the negative terminal — whilst the positive terminal 

 is a central carbon rod; and the newer 'layer' type, reminiscent of the 

 original voltaic pile, in which the cells are flat slabs comprising a 3-deck 

 sandwich as in Figure 19.1. 



Cylindrical cells are used where the discharge current is relatively high, 

 and layer cells when the current is comparatively low. Thus the former are 

 used for LT supplies, flash lamp bulb operation, etc., also HT batteries of 

 the older pattern. Layer type batteries are intended for HT supplies to 

 battery radios, hearing aids, and electronic equipment. 



Though there are an enormous number of types of dry battery, they are 



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