746 SCIENCE AND HYPOTHESIS 



displacement on the galvanometer. That is true in a sense. What 

 he has shown directly is that electro-magnetic induction is not in- 

 stantaneously propagated, as was supposed, but its speed is the speed 

 of light. Yet, to suppose there is no current of displacement, and 

 that induction is with the speed of light; or, rather, to suppose that 

 the currents of displacement produce inductive effects, and that the 

 induction takes place instantaneously comes to the same thing. 

 This cannot be seen at the first glance, but it is proved by an analysis 

 of which I must not even think of giving even a summary here. 



V. Rowland's Experiment. But, as I have said above, there are 

 two kinds of open conduction currents. There are first the currents 

 of discharge of a condenser, or of any conductor whatever. There 

 are also cases in which the electric charges describe a closed contour, 

 being displaced by conduction in one part of the circuit and by con- 

 vection in the other part. The question might be regarded as solved 

 for open currents of the first kind; they were closed by currents of 

 displacement. For open currents of the second kind the solution 

 appeared still more simple. 



It seemed that if the current were closed it could only be by the 

 current of convection itself. For that purpose it was sufficient to 

 admit that a " convection current " i.e., a charged conductor in 

 motion could act on the galvanometer. But experimental confirma- 

 tion was lacking. It appeared difficult, in fact, to obtain a sufficient 

 intensity even by increasing as much as possible the charge and the 

 velocity of the conductors. Eowland, an extremely skilful experi- 

 mentalist, was the first to triumph, or to seem to triumph, over these 

 difficulties. A disc received a strong electrostatic charge and a very 

 high speed of rotation. An astatic magnetic system placed beside the 

 disc underwent deviations. The experiment was made twice by Eow- 

 land, once in Berlin and once at Baltimore. It was afterwards re- 

 peated by Himstedt. These physicists even believed that they could 

 announce that they had succeeded in making quantitative measure- 

 ments. For twenty years Eowland's law was admitted without ob- 

 jection by all physicists, and, indeed, everything seemed to confirm it. 

 The spark certainly does produce a magnetic effect, and does it not 

 seem extremely likely that the spark discharged is due to particles 

 taken from one of the electrodes and transferred to the other electrode 

 with their charge? Is not the very spectrum of the spark, in which 

 we recognize the lines of the metal of the electrode, a proof of it? 

 The spark would then be a real current of induction. 



On the other hand, it is also admitted that in an electrolyte the 

 electricity is carried by the ions in motion. The current in an electro- 

 lyte would therefore also be a current of convection; but it acts on 

 the magnetic needle. And in the same way for cathodic rays ; Crooks 

 attributed these rays to very subtle matter charged with negative elec- 



