the Induction- Coil. 717 



The oscillograph was standardised in this and the two 

 following cases, not directly with the air-core coil, but with 

 the coil and condensers used in Case III., the greatest deflexion 

 for a current of 8*5 amperes in that case being taken as 

 representing 26500 volts. A comparison of the curves leads 

 to 39480 volts as the greatest observed value of the secondary 

 potential, which is again considerably below the calculated 

 value. 



The sparking-di stance between the two spheres with the 

 same primary current was found to be 1*6 centimetres. 



Case VI. — In order to approximate more closely to the 

 ordinary working conditions of an induction-coil, the pri- 

 mary capacity was now considerably reduced in value. This 

 has the effect of increasing the secondary potential, and at 

 this stage it was noticed that at high potentials a brush 

 discharge took place freely between the ebonite sheath 

 enclosing the attracting plate of the oscillograph and other 

 parts of the instrument. The attracting plate and its con- 

 necting rod were therefore provided with still thicker coverings 

 of ebonite, and the terminals of the instrument were removed 

 to a greater distance from each other, in order to diminish as 

 far as possible the leakage due to this cause. 



The primary capacity was in the present case 3*89 micro- 

 farads, which gives, by (2), using the values given under 

 Case V. for the other constants of the circuits, 



ti, = 174-4, w 2 =1371. 



The expression for the secondary potential in volts is 



2V 2 = 21650 ?>- 4 ' 14 * sin (62770^-0-1) 



- 2753 to*- 65 " 26 ' sin (493452* - 0*83), 



where i is the primary current in amperes. The curve 

 representing the square of this expression was drawn for 

 7*0 = 2-25 amperes, and is shown in fig. 6 (p. 718). 



Photographs were taken at the interruption of various 

 currents in the primary circuit. One of these is shown in 

 Plate XIII. fig. 13. The curves obtained were all of the 

 same type, only the amplitude varying with the primary 

 current. The greatest ordinate in each photographic curve 

 was measured and compared with that of the standardising 

 curve. The results are contained in Table I., in which the 

 first column contains the values of the current interrupted in 



Phil. May. S. 6. Vol. 22. No. 131. Nov. 1911. 3 B 



