SCIENCE AND PRACTICE. 303 



the needle is observed to swing through an angle of < degrees. 

 After a lapse of one minute, or other given interval, the 

 stopper L L' is replaced again, and the swing ^J due to this 

 second charge current observed. 



The quantity of electricity which is measured by the second 

 observation is obviously not that which remains in, but that 

 which has escaped from the cable ; in other words, we make 

 good the loss which the charge has sustained in the time 

 between the observations, and this loss, L, is in terms of the 

 whole charge, 



0, 

 sin. ' 



2 



L = "1 5 

 sin. -L- 



49. Constant of Sensibility of the Galvanometer. Galvano- 

 meters with single magnets do not vary their constants of 

 sensibility considerably unless it is very soon after the mag- 

 netising of the needle, but those with astatic systems are 

 very inconstant, altering sometimes during an observation. 



For all calculations from measurements by deflection the 

 constant of sensibility must be known. This constant is 

 the deflection produced when an unit of electro -motive 

 force is in circuit with an unit of resistance. The unit of 

 resistance used in measuring insulation of cables is one 

 million, or 10 6 times the small unit used in measuring 

 metallic resistances. This small unit is the resistance of a 

 column of mercury a meter long and a square millimeter 

 transverse section ; the great multiple of it is not called an 

 unit, but simply a " million," and the insulation of a cable 

 is expressed as having so-and-so many millions, meaning so- 

 and-so many million times the little column of mercury. 



A million metres of mercury, or its equivalent resistance 

 in any other metal, would be very difficult to employ, and we 

 are happily prevented the necessity of employing it to obtain 

 the same deflection of sensibility by using a shunt and a 

 smaller resistance. 



In the circuit represented in Fig. 142 the current of the 



