and its Effects. 205 



mutual action of the coils of wire, partly also and principally 

 through the influence of the soft iron core in connexion with the 

 interrupting hammer. 



For this purpose it is best to remove the induction coil for the 

 time, and to retain only the primary coil in connexion with the 

 galvanic battery and the hammer. 



If, in the first place, the wires of the primary coil be connected 

 either side by side, or one after the other, in such a manner that 

 the current must ti-averse them in opposite directions, instead of 

 in the same dii'ection, as is usually the case, the production of 

 the extra current will be effectually prevented, and on this account 

 only a very small spark of a purely galvanic nature is observed 

 at the point of interruption where the hammer vibrates in a 

 vacuum*. The insertion of a soft iron core into the primary 

 coil has no influence upon this result, for the iron, as will be 

 readily seen, is not magnetized. The addition of a condetiscr is 

 equally without efiect. 



Passing from this combination to that wherein the current 

 traverses the wires in the same direction, the influence of the 

 extra current shows itself immediately and in a remarkable 

 manner. The scarcely perceptible galvanic spark expands itself 

 into a luminous phsenomenon, whose magnitude is actually sur- 

 j)rising when we consider that it is obtained from a wire only 

 100, or at most 200 feet long. Notwithstanding its magnitude, 

 however, it is explained by the fact, that, previous to its deve- 

 lopment, the above wire constituted a complete circuit to a cur- 

 rent of comparatively great intensity. 



In general this phfenonicnon consists of two parts : a more or 

 less luminous spark at the place where the current is interrupted, 

 and a soft, clear, blue light surrounding the neighbouring parts 

 of the hamjner. The formation of both, however, is different 

 according as the galvanic current is excited by one, two, or three 

 of Grovels elements ; according as the wires are connected side by 

 side or one after the other, or a soft iron core inserted in the coil 

 or not ; and lastly, according as a condenser is used or not. I 

 have generally employed a battery of two of Grove's elements, 

 and connected them with the hanuncr so that the pin formed the 

 negative pole. Under these circumstances the following was 

 observed . 



By combining the primary wires side by side, the spark is 

 pretty strong, but the blue light very weak, provided no soft 



* This method of connecting the wires is perhaps the only practical 

 means of excluding the influence of induction currents when measuring the 

 velocity with which electricity is propagated ; an influence to wliich must 

 be chiefly ascribed the exiraordinary disparity l)etwe(;ii the results already 

 obtained. 



