Batteries for supplying small currents. 



331 



A convenient feature of the battery-box shown in the illus- 

 tration is the ease with which the cells can be removed and 

 replaced by new ones when necessary. The cells as supplied 

 by the makers are cylindrical in shape, the outside being of 

 zinc which is the negative pole of the cell. A small brass 

 cap connected to the positive pole protrudes from the centre 

 of the top of the cell. The cells are each about 5 cm. high 

 and 1'4 cm. in diameter. They are arranged in a circle 

 between two sheets of ebonite, one of which forms the top of 

 the box and the other is inside the box and is supported from 

 the top by four ebonite rods. Each cell is held in position 

 by two copper springs arranged as represented in fig. 3. 

 The cell A rests on the lower ebonite sheet (^ inch thick), 



Ficr. 3. 



and is gripped by the spring clip B which is supported from 

 the brass base E screwed to the lower ebonite sheet. The 

 brass cap of the positive pole of the cell presses against the 

 copper spring C fixed to the brass piece D which is screwed 

 to the under side of the upper sheet of ebonite (J inch thick) 

 which forms the top of the box. The upper brass piece D, 

 connected to the positive pole of the cell, is joined by an 

 insulated wire to the lower brass piece (corresponding to E) 

 in connexion with the zinc of the next cell. This method 

 of connexion is continued round the circle, so that all the 

 cells are joined in series. Inside the box a radial arm makes 

 a rubbing contact with the under side of D, and the position 

 of the arm is indicated by the pointer which moves over the 

 dial on the top of the box. This arm is connected to the 

 left-hand terminal seen in fig. 2 ; the other terminal is con- 

 nected to the zinc of the first cell in the series. It has been 

 found convenient in practice to have one position of the 

 pointer in which there is no connexion between the ter- 

 minals (" off "). This forms a simple method of breaking 

 the battery circuit. It is also convenient for some purposes 

 to have a position in which the terminals are connected, but 



