328 ProL". E. Taylor Jones on the Secondary 



adjustments which are most favourable to the conversion of 

 magnetic energy of the primary current into electrostatic 

 energy in the secondary circuit *. In one of these adjust- 

 ments the coefficient of coupling is determined by k 2 = 0'57 y 

 the primary capacity by L 1 C 1 = , 43 L 3 G 2 . If the primary 

 is drawn out to this position, and the capacity of the primary 

 condenser adjusted to its optimum value, the electrostatic 

 energy generated in the secondary circuit (for a given value 

 of the primary current) will be at its maximum. The 

 position of the primary which gives maximum secondary 

 potential will probably be influenced to some extent by the 

 reduction of capacity of the secondary coil due to the dis- 

 placement of the primaryf. 



In the case of the 18-inch coil above referred to, whether 

 worked by the slow break or by a motor interrupter, the 

 spark-length at break is distinctly increased by drawing out 

 the primary to a distance of 30 cm. from its symmetrical 

 position. It is worthy of note that if an X-ray tube is con- 

 nected to the secondary terminals the mean current through 

 the tube, as indicated by a moving-coil milliampere meter, 

 is less, but a fluorescent screen on which the rays fall glows 

 more brightly, in the displaced than in the symmetrical 

 position. 



Thus the process of reducing the mutual inductance by 

 drawing out the primary coil to a suitable distance along the 

 axis of the secondary not only materially reduces the negative 

 secondary potential at make, but also increases the positive 

 potential at break. 



According to (15) the secondary potential at make is, 

 other things being the same, inversely proportional to L lf the 

 self-inductance of the primary circuit, and may be reduced 

 by connecting an inductance-coil in series with the primary 

 and the battery. Thus the addition of an air-core coil of 

 about 0'07 henry to the primary circuit of the 18-inch coil, 

 the total resistance of the primary circuit (10 ohms) being 

 kept constant, reduced the make-spark from 4' 5 mm. 

 to 2*7 mm. The effect is even more marked when the four 

 layers of the primary coil are connected in parallel, h f being 

 now much smaller to begin with. The insertion of the air- 

 core coil now reduces the spark-length at make from 13'1 

 to 1*0 mm. 



* Phil. Mag. Jan. 1915, pp. 1-4. 



'!' The same effect. — a maximum of secondary potential at break in a 

 certain displaced position of the primary coil — is also observed when the 

 secondary terminals are connected to a condenser, in which case the 

 change of C 2 due to the displacement of the primary is insignificant. 



