220 



Messrs. Trowbridge and Duane on 



quoted and 2*998 X 10 10 , which is generally accepted as the 

 velocity of light. Since then we have succeeded in producing 

 much better waves and much more regular sparks, and have 

 discovered a phenomenon which renders a measurement on 

 a photograph over a space where the dots are obliterated a 

 questionable proceeding. The new data have given a value 

 for the velocity more in accord with theory. 



Since the waves in the secondary were not well formed 

 when the spark-gap was inserted, it seemed desirable to try 

 to find an arrangement that would produce simultaneously a 

 good wave and a photographable spark. A number of con- 

 densers with plates of different sizes and shapes and different 

 substances for the dielectric were tried, and the apparatus to 

 be described was finally adopted. The difficulties to be over- 

 come were these. Too strong a reaction between the primary 

 and secondary condensers could not be employed, because the 

 increase in the damping of the primary due to the large 

 amount of energy drawn off by the secondary made good 

 resonance impossible. The amount of energy in the primary 

 at full charge must be much greater than that in the secondary. 

 On the other hand the capacity of the primary condenser must 

 not be too great ; for the self-induction of the primary circuit 

 would have to be proportionately small, and this, too, means 

 an increase in the damping. The secondary condenser, too, 

 must have a capacity of less than a certain magnitude in order 

 that the node may fall on the circuit and not in the condenser 

 plate. These points seem to indicate that small condensers 

 are preferable to large ones ; but a decrease in the size of the 

 plates means a decrease in the light of the secondary sparks, 

 and the sparks are at best barely photographable. Practically, 

 therefore, our choice was much limited, and the particular 

 arrangement to give the best results had to be selected by 

 experiment after a long series of trials. The arrangement and 

 dimensions of the apparatus finally adopted were as follows : — 



Two metallic plates, a and b (fig. 4), 30 x 30 centim., placed 



Fig. 4. +' 



B 



SlL 



H 



in vertical planes, formed the primary condenser. The 

 dielectric between them consisted of the best French plate- 

 glass obtainable (K = 8+ probably) and was 2 centim. thick. 



