132 Construction and use of electrometer 



electrical machine was turned till the balls appeared even with those lines. 

 By these means I could judge of the strength of the electricity to a con- 



rt 



siderable degree of exactness. In order to make the straws conduct the 

 better they were gilt over. 



250] In order to estimate what error may arise from the vials being 

 not equally charged in both trials, let the required surface of the trial 

 plate be called 16; then must the surface which must be given to it in 

 order that the balls may separate negatively be 20, or 16 + 4, supposing 

 (he vials to be charged with the usual degree of strength. Suppose now 

 that in the next trial, in which the balls are to separate positively, the 

 vials are charged stronger than before, in the ratio of x to i, so that the 

 quantity of redundant fluid in B shall be greater than before, in the 

 ratio of x to i, and that the deficience in the trial plate should be greater 

 than before in the same ratio, provided its surface remained unaltered; 

 then must the surface which must be given to the trial plate, in order 

 that the balls shall separate positively as much as they did negatively, 



be 16 - -; for, if this surface is given to it, it is plain that the redundant 



X 



fluid in B will as much exceed the deficient in the trial plate as it before 

 fell short of it. The mean between these two surfaces is 16 + - 



2X 



whereas it ought to have been 16, so that the error which will proceed 

 from thence in finding the required surface of the trial plate is - 



and, consequently, is less than half of the error which we are liable to in 

 finding it the other way (or that in which we endeavour to find that 

 surface of the trial plate with which the balls do not separate at all), 



