192 ELEMENTARY LESSONS ON [CHAP. iv. 



other units can be derived from these, as is explained 

 in the Note at the end of this Lesson. Now, amongst 

 the derived units of this system is the unit of force, 

 named the Dyne, which is that force which, acting for 

 one second on a mass of one gramme, imparts to it 

 a velocity of one centimetre per second. Taking the 

 dyne then as the unit of force, and the centimetre as 

 the unit of length (or distance), we must find a unit of 

 electric quantity to agree with these in our equation. 

 It is quite clear that if q, q ', and d were each made equal 

 to i (that is, if we took two charges of value i each, 

 and placed them one centimetre apart), the value of 



( I x 9 u i i x i 



~~^T~ would be , which is equal to i. Hence we 



adopt, as our Definition of a Unit of Electricity, the 

 following, which we briefly gave at the end of Lesson II. 

 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. 



An example will aid the student to understand the 

 application of Coulomb's law. 



EXAMPLE. Two small spheres, charged respectively with 

 6 units and 8 units of + electricity, are placed 4 

 centimetres apart ; find what force they exert on one 



another. By the formula, /= -^/-> we find / = 



2~ ^ 3 dynes. Examples for the student 

 are given in the Questions at the end of the Book. 



The force in the above example would clearly be a force 

 of repulsion. Had one of these charges been negative, 

 the product q x q would have had a value, and the 

 answer would have come out as mimis 3 dynes. The 

 presence of the negative sign, therefore, prefixed to a 

 force, will indicate that it is a force of attraction, whilst 

 the -f sign would signify a force of repulsion. 



237. Potential. We must next define the term 

 potential, as applied to electric forces ; but to make 



