188 ELEMENTARY GENERAL SCIENCE CHAP. 



the zinc weighed less in Expt. 181, after the electric current 

 had flowed for some time along the joined wires. 



It is customary to give the name electro-motive force to the 

 difference of potential which we have seen exists between the plates 

 of copper and zinc. As the student will learn when he continues 

 his study of the electric current, this difference of potential, or 

 electro-motive force, can be measured by means of an instrument 

 devised by Lord Kelvin, called the Quadrant Electrometer. 1 

 But though the term electro-motive force is used to speak of 

 this difference of potential, it is not a force in the sense in which 

 the word was used in Chapter V. A force is that which causes 

 motion in matter, but an electro-motive force results only in a 

 motion of electricity, which is not matter at all. 



Voltaic Cells. The simplest arrangement for developing an 

 electric current by means of chemical action has been now de- 

 scribed, but there are many others which are much better. They 

 are all called "voltaic" cells after the name of the physicist Volta, 

 who was the first to develop an electric current by chemical means. 



To be of any practical value the voltaic cell must not cease in a 

 short time to give a current as the simple device adopted in 

 Expt. 186 did, as soon as bubbles of gas had collected over the 

 copper plates. Some means have to be taken to prevent this 

 stopping of the current by the bubbles of hydrogen, or, as it is 

 called, the polarisation of the cell. 



Polarisation is prevented in two ways : (1) by mechanical, (2) 

 by chemical means. 



Prevention of Polarisation by Mechanical Means. Smee's 

 Cell. Smee substituted a plate of silver on which platinum had 

 been deposited, that is, a plate of platinised silver, for the copper 

 plate in Volta's simple cell. The platinum on the silver plate 

 causes it to be very rough, with the result that there is a con- 

 tinual tendency for the bubbles of gas to rise from the points of 

 roughness, up through the liquid, and to escape at the surface. 



In other patterns of cell a mechanical device for keeping the 

 liquid stirred near the copper plate is adopted, the agitation of 

 the liquid being quite enough to prevent the collection of gas 

 bubbles. 



Chemical Changes in a Simple Cell. To understand clearly 

 the principle of the construction of cells in which the difficulty of 

 polarisation is got over by chemical means, it will be necessary to 



1 See Prof. S. P. Thompson's Electricity and Magnetism, p. 272. 



