244 LECTURES TO SCIENCE TEACHERS. 



between them draws the plate c down, and the force with 

 which c is attracted downward depends on the difference 

 of potentials between the plates A and c, and on the distance 

 between them. The plate A can be raised or lowered by 

 means of a screw, and, when an experiment is being made, 

 its distance from c is adjusted till the index hair is in 

 the sighted position. When that is the case it is known 

 that the attraction between the plates is equal to the force 

 of gravity on the weight previously determined. 



Now it was shown by Sir William Thomson in one of his 

 earliest papers on electricity, a paper in which he considered 

 the validity of the objections raised by Snow Harris against 

 the laws of Coulomb, that in a case such as we have been 

 considering, where the lines of electric force are straight 

 lines between two attracting discs, the force, F, of attrac- 

 tion, will be given by the formula 



in which v is the difference of potentials of the two plates, 

 s the surface of the smaller, and D the distance between 

 the two. From this we obtain at once 



Hence if we measure the area s, the distance D, and the 

 force F, which, as I have said, is done by the previous 

 weighing, we can at once calculate v ; and it is to be 

 noticed that if F is expressed in absolute units of force, 

 the numerical value of v deduced by this formula will 

 be obtained in absolute electrostatic units of potential. 



In working practically with the absolute electrometer, how- 

 ever, Sir William Thomson found great difficulty in measur- 

 ing D, the distance between the plates, with sufficient 

 accuracy. He therefore adopted a mode of employing the 

 instrument somewhat different from that which I have in- 

 dicated. The plate A is connected with a separate electro- 

 meter, and with a Leyden jar and replenisher, and is main- 

 tained at a constant high potential. To measure the 

 potential of any conductor, the plate c is first connected 

 with the earth, the plate A is adjusted till the hair is in the 

 sighted position, and a reading taken, called the " earth 



