Objects, and on the Oscillating Electric Spark. 259 



number expressing C is too small by 1*33 per cent., therefore 

 the number expressing L is also too small by 1'33 per cent., 

 and the nominal value of L must be multiplied by 1*0133. 

 But if r t and r 2 are given in B.A. units, then, since these 

 numbers are each too large by 1*33 per cent., therefore the 

 number expressing L is also too large by this amount, since 

 one of the resistances and the capacity will balance, and 

 therefore the number expressing L must be divided by 1*0133. 

 Now, the decrement being small, the numbers of oscillations 



in a second n= ■= = ; therefore, if L and C have each to 



2ttv/LO ' 



be multiplied by l c 0133, n should be divided by 1*0133; but, 

 on the other hand, if one has to to be multiplied and the other 

 divided by 1*0133, then these corrections balance and n 

 requires no correction. 



Messrs. Robson and Smith determined from the photo- 

 graphs the actual number of oscillations per second, and they 

 determined also, assuming the microfarad to be correct, the 

 number that ought to have been produced. The following 

 are their results : — 



Observed number . , . 2293 3170 3264 2115 1806 

 Calculated number . . 2210 3126 3126 2210 1826 

 Number of elementary^ 



sparks in complete > 16 23 20 15 14 



spark J 



These were all taken on half plates and were subject to the 

 difficulty of centering. One spark was taken on a large 

 plate, so that all six lenses produced images of the same spark. 

 In this case the observed number was 1852, and the calculated 

 number 1826. Now, if this number is corrected by dividing 

 it by 1*0133, the calculated number becomes 1802, showing 

 the actual number to be about 2*7 per cent, too great. As 

 window-glass was used for the condenser, which, owing to 

 soakage, gives smaller capacities as the time of discharge is 

 made smaller, it is probable that part of the discrepancy may 

 be due to this ; but in the measurement of the capacity of the 

 condenser the effect of soakage was made as small as possible 

 by making, as far as such a thing could be done by hand, the 

 contact of discharge instantaneous. The self-induction, too, 

 as measured, may have been slightly different to the self- 

 induction for oscillations at the rate of 1852 a second. 



Mr. Smith has calculated the self-induction, using Prof. 

 Perry's formula, given at the last meeting of the Physical 

 Society, but the irregular form of the coils would not permit 



