^Batteries for supplying small currents. 



329 



The advantage of a battery which does not require 

 periodical charging is obvious, and several types of primary 

 cell have been used for this purpose. Primary cells are 

 usually very satisfactory for electrometer work and for 

 experiments where potential only, and not current (or only a 

 very minute one) is required. They are often troublesome to 

 fit up, but require no further attention if treated carefully. 

 For currents of the order of "01 ampere dry cells may be 

 used; and the writer has found those made by the British 

 Ever-Ready Co. very convenient for this purpose. These 

 cells have the advantage of being small, thus enabling a 

 large number to be packed into a small space, and their 

 electromotive force falls but slowly when currents of only a 

 few milliamperes are taken from the battery, so that the 

 cells can be used for a long time before it is necessary to 

 replace them. 



Owing to the impossibility of obtaining a further supply 

 of lead plates for small storage cells, the writer was induced 



Fig. 1. —200 Volt Battery. 



to experiment with dry cells early in the present year 

 These were arranged in batteries of two forms which have 

 been found so convenient and satisfactory^ in use that a 

 description of them may be useful to others. 



The battery illustrated in fig. 1 is for supplying potentials 

 up to about 200 volts. The cells are contained in a wooden 

 box 61 cm. long, 18*5 cm. wide and 11*5 cm. high. This has 

 an ebonite plate on the top which insulates the plug-keys 

 by means of which the cells are arranged in series. The 

 cells used are the Ever-Ready dry battery No. 1689 of 

 about 4*0 volts. It is advisable not to have too many cells 



