€2 Mr. W. Morris Jones on the most Effective 



Summary. 



The absorption spectra of the vapours of ammonium 

 chloride, mercuric and mereurous chloride, cadmium chloride, 

 bromide, and iodide were examined, and it was found : — 



(a) That the vapours did not give any well-defined 



absorption lines or bands like bromine and iodine in 

 the region X 2500-X 6700. 



(b) That they gave a general selective absorption in the 



ultra-violet (except possibly NH 4 G1) which in the 

 case o£ the cadmium salts was greater for the iodide 

 and bromide than for the chloride. 



The author wishes to thank Sir Ernest Rutherford for 

 placing the necessary facilities at his disposal, and for the 

 kind interest he has taken in the work. 



Manchester University, 

 Dec. 1915. 



"VII. On the most Effective Primary Capacity for Tesla Coils. 

 By W. Morris Jones, B.Sc, Research Student of the 



University of Wales* '. 



IT is generally assumed that a Tesla coil gives the best 

 effect (i. e. the highest secondary potential for a given 

 primary discharge potential) when the two circuits are so 

 adjusted that, separated, their periods of oscillations are 

 equal. This condition is expressed by the relation L 1 C 1 = L 2 C 2 , 

 ^and is commonly called the condition for " resonance " or 

 " synchronism," though it does not mean that the two periods 

 of the system are equal when the circuits are closely coupled. 

 The above condition appears to have been first arrived at by 

 Drudet in a well known memoir on the Tesla coil, and has 

 received general acceptance. 



In a recent paper Professor Taylor Jones t has shown that 

 the above condition does not apply if the adjustment is made 

 by varying the primary capacity alone, but that in this ease 

 the " optimum " value of Ci is considerably greater than the 

 resonance value. Taking the secondary potential, when the 



* Communicated by Prof. E. Taylor Jones, D.Sc. 

 f Ann. de Physik, xiii. p. 512 (1904). 

 J Phil. Mag. xxx. p. 224 (1915). 



