338 Mr. R. Sabine on a Method of Measuring 



necessary to know exactly the value of the resistance ; and as 

 this could not be depended upon if the discharge took place 

 through the dielectric, a known wire resistance, r, has to be 

 inserted between the two sides of the accumulator, so that the 

 time during which the potential P of the charge falls to p is 



/I P 



4- seconds. 



The internal or insulation resistance R of the accumulator 

 being made so great in comparison with r that it may practi- 

 cally be regarded as infinite *, the time is then expressed by 



p 



t=frlog ? — seconds. 



Assuming that the excursion of the light-point of a mirror- 

 galvanometer is proportional to the quantity of electricity sud- 

 denly discharged through its coil, if the excursions are respec- 

 tively C and c, the time is 



C 

 t=fr\og e — seconds. 



The only values therefore required to be known are the capa- 

 city (/) of the accumulator, and the resistance (r) of the dis- 

 charging ware. The measurement is independent of the con- 

 stant of sensibility of the galvanometer and of the electromo- 

 tive force of the battery, neither of which, however, must vary 

 during an observation. 



I have employed this system to determine the intervals of 

 time which elapse 



(1) Between the successive interruptions of two circuits, 

 and 



(2) Between making and breaking a single circuit. 



The interval of time to be measured is, of course, always that 

 during which the accumulator is allowed to partially empty 

 itself through the discharging resistance. 



When the time to be measured is that which elapses between 

 the successive interruptions of two circuits, the apparatus is 

 arranged in principle as is shown in fig. 1. The two circuits 



* With, a mica accumulator of ^ microfarad capacity, the discharging 

 resistance, r, necessary for determining intervals of time between t -Jq- and 

 iV of a second is under 1 megohm. The internal resistance of such an ac- 

 cumulator is considerably over 200,000 megohms, so that practically it 

 may be assumed to be in all cases infinite in comparison with r. 



