1$ ELEMENTARY LESSONS ON [CHAP. i. 



Before leaving the subject of electric forces, it may be 

 well to mention that the force of attraction between 

 two oppositely electrified bodies varies also inversely as 

 the square of the distance between them. And in every 

 case, whether of attraction or repulsion, the force at any 

 given distance is proportional to the product of the 

 two quantities of electricity on the bodies. Thus, if 

 we had separately given a charge of 2 to the ball m and 

 a charge of 3 to the ball n t the force between them will 

 be 3 x 2 = 6 times as great as if each had had a 

 charge of i given to it. 



17. Unit Quantity of Electricity. In conse- 

 quence of these laws of attraction and repulsion, it is 

 found most convenient to adopt the following definition 

 for that quantity of electricity which we take for a unit or 

 standard by which to measure other quantities of elec- 

 tricity. One Unit of Electricity is that quantity which, 

 when placed at a distance of one centimetre from a 

 similar and equal quantity, repels it with a force of 

 one dyne. Further information about the measure- 

 ment of electrical quantities is given in Lessons XX. 

 and XXI. 



LESSON III. Electrification by Induction. 



18. We have now learned how two charged bodies 

 may attract or repel one another. It is sometimes said 

 that it is the electricities in the bodies which attract or 

 repel one another ; but as electricity is not known to 

 exist except in or on material bodies, the proof that it is 

 the electricities themselves which are attracted is only 

 indirect. Nevertheless there are certain matters which 

 support this view, one of these being the electric influ- 

 ence exerted by an electrified body upon one not 

 electrified. 



Suppose we rub a ball of glass with silk to electrify it, 



