Chap, ii.] ELECTROMOTIVE FORCE. 17 



plate which it joins. Tn one case the difference may 

 be a + quantity, in another a quantity ; but the 

 sum of the differences gives what is called the electro- 

 motive force of the element. 



In the element, the chemical changes going on 

 between the plates and the liquid are the source of 

 the energy which enables the element to do work ; 

 that is, the energy of the element may be measured 

 by the chemical decompositions going on in it. 



Electromotive force is another phrase for 

 "difference of potential." For measuring electro- 

 motive force a standard is adopted, just as for 

 measuring the weight of a body a standard, viz. the 

 pound, is employed. The standard or unit of electro- 

 motive force is called the VOLT, after Yolta, It is said 

 to be '9268 of the electromotive force given by a 

 Daniell cell. (See page 19.) That is to say, a Daniell 

 gives 1'079 volts. 



Poles, or electrodes. It has been noted that 

 the part of the metallic plates projecting above the 

 fluid of the element is termed the pole, positive or 

 negative as the case may be. If wires be attached to 

 these parts for conducting the electricity to some 

 distance they are also called poles (+ or -). They 

 are also termed ELECTRODES (??Ae/cTpoz/ ? and <55bs, a way) 

 because they are the pathways along which the 

 electricity travels. The wires used may be made 

 of any length so as to convey the electricity to a 

 distance from the place of generation. They must, of 

 course, be made of good conductors. Copper is a good 

 conductor, and is preferred for its cheapness. The 

 electrodes are usually protected by a coating of cotton, 

 thread, silk, or guttapercha, all of which are insulators, 

 and prevent the electricity being led off by contact 

 with other bodies. 



Polarisation of plates. It has been already 

 noted that in the voltaic element hydrogen is liberated. 

 c 7 



