290 Records of Measures 



M. 3. The charge of each row of the battery was found by charging to a 

 given degree by electrometer and touching it repeatedly with jar 4 till the 

 separation of electrometer was reduced to that answering to \ the charge. 



*The I st , 2 nd , 3 rd , 4 th , 5 th , 6 th , 7'" row required to be touched 18, 19, 17, 

 i8, 17, 17, 18 times, therefore charge 



I st row = 26 charge of jar 4 



2 nd = 27-4 



3'd = 24-6 



4"> = 26-7 



5" = 24-6 



6 = 24-6 



7 th = 26 



and charge of whole battery = 180 times that of jar 4 



and real charge = 321000 



and if real charge by computed of white glass = 7-5, 



computed charge = 42800 

 which answers to 187 square feet of glass whose thickness = T ' ff . 



Therefore charge of jar 4 answers to 1-04 square feet of D thickness. The 

 coating is about T ^- of a square foot, and therefore the mean thickness = -058. 



582] M. 4. Let jar be touched n times f by jar which is to first as x to i, 

 it will be reduced in ratio of i to (i + x) n , therefore if it is reduced to \ thereby 



(l + x) n = 2. 



Therefore let N. L 2 = a and N. L (i + x) = px, 



pxn = a, 



i pn 

 and 



x a 



x^ / x\ 

 but N. L (l + x) = x nearly, = x ( I J , 



therefore p = i - - nearly, = i - - nearly, 



therefore - = - ( i ) nearly, 



x a \ 2nJ 



= " - J nearly, 

 whence we have the following 



Rule for finding ratio of charge of 2 jars, supposing the charge of first to 

 be reduced to J by touching n times by 2 nd . 



Charge of I st is to that of 2 nd :: i '44471 - \ to i. 



* N.B. The left-hand row is supposed to be called the I st row. [If Jar 4 = 2675 

 circ. inc. (see Art. 506) whole battery = 481,500 circ. inc. or 321,000 glob, inc., 

 counting i glob. inc. = 1-5 circ. inc., as Cavendish seems to do here.] 



t [Art. 413.] 



