HERTZ .S RESEARCHES ON ELECTRICAL WAVES. 



181 



90 degrees from tlie base-line. The direction of the intoiference at the 

 null i)oint, which has already been considered, was taken as negative, 

 the interference being considered positive wlien it was [)rodnced by the 

 passage of waves on the side of C remote from P, which makes the signs 

 correspond with those of the previons experiments. It mnst be borne 

 in mind that the direction of the resultant e.m. F. at the null point is 

 oi)posed to that of the E. m. f. of induction, and therefore the first table 

 w^ould have begun with a negative sign if the electro-static E. M. F. could 

 have been eliminated. The present experiments showed that ui) to a 

 distance of 3 meters interference continued to occur, and always of the 

 same sign as at the null point. It was unfortunately impossible to ex- 

 tend these observations to a greater distance than 4 meters, on account 

 of the feebleness of the sparks, but the results obtained were sufficient 

 to give distinct evidence of a finite velocity of propagation of the E. m.f. 

 of induction. These observations, like the former ones, were repeated 

 with various lengths of the wire m n in order to exhibit the variation in 

 I)hase, and the results obtained are given in the table below : 



Table HI. 



which shows that as the distance increases the phase of the interference 

 changes in such a manner that a reversal of sign takes place at intervals 

 of from 7 to 8 meters. This result is further confirmed by comparing 

 the results of Table III with the results for greater distances given in 

 Table TI, for in the former series, the effect of the electro static E. m. f. is 

 eliminated, owing to the special position of the secondary circuit, while 

 in the former it becomes insensible at the greater distances, owing to 

 its rapid decrease with increasing distance. We should therefore ex- 

 jiect tlie results given in the first table for distances beyond 4 meters 

 to follow w'ithout a break the results given in Table III for distances 

 up to 4 meters. This was found to be the case, as is evident from in- 

 spection of Tables II and III. 



To show this more clearly the signs of the interference of the waves, 

 due to the electro-dynamic e.m.f., with the waves in the wire, are col- 

 lected together in Table IV, the first four columns of which are taken 

 from Table III, and the remaining columns from Table 11. 



