582 Prof. E. Taylor Jones on 



the best effects would be obtained without any condenser. 

 The above theory, however, points to a maximum spark- 

 length with a certain condenser, at any rate in the case 

 of negligible resistances, even with a perfectly sudden 

 interruption. 



The following experiments were therefore undertaken 

 with a view to testing this point by observing the lengths 

 of spark obtained with various capacities and different forms 

 of interruptor. The condenser used was a subdivided mica 

 microfarad, in which the fractions could be connected either 

 in parallel or in cascade. This was in each case connected 

 directly across the interruptor. The electrometer, though 

 not used in these experiments, was left connected to the 

 secondary coil in parallel with the ball electrodes, so that 

 L 2 C 2 and 7c 2 had the same values as in the experiments 

 described above. 



Experimenting first with the mercury interruptor and one 

 secondary cell (i about 1*1 amperes), no amperemeter or 

 rheostat being included in the circuit, it was found that the 

 mica condenser was as a rule sufficient to prevent any 

 visible spark at the interruptor, even when all its sections 

 were in cascade. There was occasionally a spark at the 

 interruptor but no account was taken of the result when this 

 occurred. The capacities were taken in pairs, each two 

 being tried alternately many times at the same distance. 

 Thus '5 mfd. was found to give a longer spark than 1 mfd., 

 *3 longer than "5. The longest spark was given by "05 mfd. 

 Special attention was given to this capacity and '025 mfd., 

 which were tried on many occasions. A distance could 

 always be found at which '05 gave a spark every time, 

 while *025 always failed. The distance was not the same on 

 all occasions but v*as generally about 19*5 mm., and the 

 difference of the sparking distances for these tw T o capacities 

 was about 1*5 mm. A longer spark was given by *05 than 

 by *1 mfd., while *025 and "02 did not show any appreciable 

 difference. A smaller capacity, in the form of an oil con- 

 denser of about *002 mfd., was tried, but in this case there 

 was always sparking at the interruptor. 



With the mercury break and one cell, therefore, the most 

 effective capacity was about "05 mfd.*, but there were' 

 indications of a second (smaller) maximum at about *3 mfd., 

 this capacity giving consistently a rather longer spark than 

 •2 mfd. 



When tried with two cells, an amperemeter and a rheostat 



* The same result was found with a motor interraptor of the centri- 

 fugal type. 



