146 ELECTRICITY 



electro-motive force. When cells are arranged in series, 

 the negative plate of each cell is joined to the positive 



plate of 'another; between 

 any two cells the wires 

 may lead away to some in- 

 strument (Fig. 106). Series 

 arrangement is the more com- 

 FIG.IOG mon; each cell added to a 



battery gives a gain of electro-motive force. 



168. Uses of Battery Currents. Battery currents are 

 not powerful enough to do heavy work. They are used 

 for call bells, door openers, spark coils for firing explo- 

 sives, electric signals, medical batteries, telegraph and 

 telephone systems, and like purposes. 



169. Electrical Measurements. Quantity of electricity 

 is expressed in terms of coulombs. Current strength^ or 

 greatness of current, is measured by amperes. The 

 electro-motive force of a current is expressed in volts, 

 while the ohm is the unit of resistance. Electrical power, 

 or the rate of doing work, is measured by watts. One 

 watt is the rate at which work is done when a current of 

 one ampere flows between two points under a pressure 

 of one volt. The value of a watt is about y^g of a horse 

 power ( 68). 



QUESTIONS 



1. What is a cell? Tell how a cell may be made. Fully 

 explain its action. What causes a current to flow from it ?. 



2. What different elements are sometimes used in cells? 

 What liquids? What sort of cell is used in telegraph systems, 

 and why ? 



