68 Mr. W. Morris Jones on the most Effective 



secondary potential (for given values of m and k 2 ) should be 

 increased by the removal of one of the primary turns. This 

 was found to be the case, the greatest distance at which the 

 tube glowed being considerably greater with the 4-turn than 

 with the 5-turn primary for the same value of P. 



It was thought desirable to reduce the results of the later 

 experiments (those made with the 4-turn primary) so as to 

 ifhow what the tube-distance d would have been if the primary 

 coil had remained at 5 turns. In making this correction it 

 was assumed that the maximum secondary potential was 

 proportional to L 21 /Li. The correction was therefore made 

 by multiplying the later values of d by the ratio of the old 

 to the new value of L^/Li. 



This was determined as follows: — The change in L x was 

 found with the help of the wavemeter, the ratio of the periods 

 of a condenser being found when its terminals were connected 

 to 4 turns and 5 turns respectively of the "dummy" coil. 

 The change in L 21 was found by ballistic galvanometer 

 experiments, the secondary being connected to the galvano- 

 meter and the deflexion observed on the reversal of a given 

 current in the primary, this being done both for 4 turns and 

 for 5 turns. 



In this way it was found that the ratio of the old to the 

 new value of L^/Lj was -f^: consequently, in the results 

 given below the values of d obtained with the 4-turn primary 

 have all been reduced by 10 per cent. 



At the beginning of the series of experiments the number of 

 turns on the secondary was 192, and this number was reduced 

 by small stages until there remained only 54. During this 

 process the induction coefficient L 21 gradually diminished, 

 very slowly at first, more rapidly as the number of turns 

 became smaller. Since it was part of the object of the 

 present experiments to determine how the maximum 

 secondary potential varies with k 2 , it was necessary to make 

 allowance for the variation of L 21 , which is one of the 

 factors to which V 2 is proportional. The value of the co- 

 efficient L 2i was therefore determined (in arbitrary units) at 

 each stage by ballistic galvanometer experiments, in which 

 the primary was connected to the galvanometer, and the 

 reversed current sent through the secondary. This was 

 done with the primary at the middle, and also at the lower 

 end of the secondary. Curves were drawn showing how 

 the deflexions varied with the number of turns on the 

 secondary. They showed that the mutual induction between 

 primary and secondary was not sensibly altered until the 

 number of turns had been reduced to 100 when the primary 



