636 Prof. A. Battelli and Mr. L. Magri on 



the frequency measured each time, was calculated in the 

 manner mentioned in § 26. In the tables given above 



the 8th column, containing the values of B/t 7y , where Q 1 is 



the heat evolved in the spark and Q 2 that evolved in the 

 metallic spiral, serves to give us the mean resistance r of the 

 spark. Though these values of r are perhaps a little smaller 

 than the true ones, on account of the dispersion of heat 

 possibly occurring along the metallic rods of the spark-gap, 

 yet our experiments will allow of the conclusion that the 

 resistance of the spark has been in most cases smaller than 

 that of the metallic part, and without any doubt always inferior 

 to 1 ohm. The measurements completed by us would ascribe 

 to the mean resistance of sparks of from 1 to 5 mm. values 

 ranging from 0*18 to 0*96 ohm, with the reservation above 

 mentioned. 



Another important fact may be deduced from our ex- 

 periments, viz., that the capacity and resistance of the 

 metallic part of the circuit remaining the same, the resistance 



derived from the ratio -^ increases very slowly as the length 



H2 



of the spark increases (see Tables I. to VII.) . This may be 

 explained, either by admitting that the main part of this 

 resistance lies at the passage from the electrode into the air, 

 or that with increase of length the section of the spark 

 increases in proportion. 



With the arrangement adopted by us one may besides 

 ascertain how the resistance of the spark depends upon the 

 period of oscillation ; and a perusal of the tables above 

 given shows that this resistance — the explosive distance and 

 capacity of condenser being the same — increases with increase 

 of period. 



The noteworthy accord between the calculated value of 

 the period of oscillation and the one given by our experiments, 

 is an evidence of the smallness of the value of the effective 

 resistance of the spark ; and another evidence of the accuracy 

 of the measurements carried out is afforded by the fairly 

 satisfactory agreement between the disposable energy of the 

 condenser and the sum of the thermic energies evolved in 

 the metallic circuit and in the spark. 



It is almost superfluous to state that all we have said 

 regarding the resistance of the spark is relative to the mean 

 values it presents during the whole time it lasts. 



