544 Mr. P. E. Shaw on 



A simple example will show the value of this method. 

 Consider two cases : — 



(1) S =0-1 mfd. 

 R,= 10,000,000 co. 

 R 2 =1000«. 



E.M.F. = l'0v. 



Then, neglecting the small steady current, the quantity- 

 passing impulsively at make is 



Q=0-lx 10" 6 x 1-0 = 10" 7 coulomb. 



(2) Suppose we used a simple circuit as in fig. 1 : — 



R = 100,000 co (this is the most attained in practice). 

 E.M.F. = l-0v. 



Then the quantity passing during 1 second after make 

 would be: — 



Q = 10- 5 x 1-0= 10- 5 coulomb. 



This is 100 times as large as for case (1) ; and if the sur- 

 faces were in contact for more than 1 second, the ratio would 

 be even larger. 



The plan in working is to reduce S and increase Rj and R 2 

 down to the limit of comfortable sounding of the receiver. 

 By this care the surfaces can be used continuously for 2 to 8 

 hours, whilst when not in use they seem to remain unchanged. 



Suppose that by the passage of electricity at the contact the 

 platinum wire rose in temperature y^ of one degree, it would 

 if 1 cm. long expand about 10~ 7 cm., i. e. one of our smallest 

 measurements. Hence it is that large currents are to be 

 deprecated. 



6. Setting the Contacts. 



Suppose the apparatus set up, and the contacts clean, and 

 that we wish to get the contact position ready for taking 

 measurements. The observer arranges the observing tele- 

 phone (which is provided with long flexible wires) on his 

 head, puts on the cells used, and goes to the left-hand of the 

 apparatus (fig. 2). He then screws up the iron cylinder with 

 its receiver till contact is heard, then he tightens the lock-nut 

 till, on gently tapping, the contact is heard to rattle ; then he 

 is sure that the surfaces are approximately near one another 

 and, probably, not touching. Then he puts on the cylinder 

 cover and all other covers, and takes a seat at the right-hand 

 end of the apparatus, where, by the motion of the pulley, he 

 can obtain the exact contact position, and observe the graduated 

 disk with the telescope. 



