PEIRCE. MANNER OF GROWTH OF A CURRENT. 



535 



When the voltage of the storage battery was reduced to 90 the build- 

 iug-up curve for a reversed current of 6.9 amperes had the form of the 

 right-hand curve (1) iu Figure 19 when the gap was closed, but of the 

 next curve (2) when the air gap was 13 mm. wide. The curves cross 

 eleven seconds after the start. For the current to attain half its final 

 strength, 3.5 seconds are required when the gap is closed, but only 1.5 

 seconds when the width of the gap is 13 mm. ; the current attains 99 

 per cent of its final value more quickly when the gap is closed than when 

 it is open. 



Figure 19. 



The leftmost curve (3) of Figure 19 belongs to a reversed current of 

 1.3 amperes when the gap was closed; the ordinates are exaggerated so 

 as to make the final value the same as for the larger current. 



When a secondary coil, consisting of a few turns of insulated wire 

 wound around the whole core of the magnet, was connected with an oscil- 

 lograph which made its record on the same sheet of paper as the oscillo- 

 graph connected with the main circuit, it was easy to get the rate of growth 

 of the induction flux in the core. Figure 20 shows building-up curves 

 when the gap was closed for a current of 1.3 amperes furnished (A) by a 

 battery of 40 storage cells and (D) by a battery of 10 storage cells; in 



