AMPfeRE 227 



means of the electroscope or electrometer, which instruments 

 Ampere now expressly states to be measurers of tension. As 

 phenomena of the electric current, on the other hand, he 

 expressly puts forward the chemical and magnetic effects of 

 the current (the corresponding development of heat was first 

 really grasped by Joule), and he proves with regard to them 

 especially, that they are not phenomena of tension, since 

 they are completely absent when the latter alone is present 

 (before the circuit is closed). The current, he says, is best 

 measured by means of its magnetic effects (which is still true 

 to-day, and for which purpose the multiplier had at that time 

 been invented in Germany) and he introduced for every 

 current measurer working by means of the magnetic effect, 

 the name 'galvanometer,' which has ever since remained in 

 use.i 



Tension, or as we call it to-day, electromotive force 

 (voltage), thus appears as a cause, and current as an effect. 

 As soon as this effect results from closure of the circuit, the 

 phenomena of tension disappear, 'or become imperceptible.' 

 Ampere even imagined the resistance of the circuit already 

 as determining the current strength at a given tension. We 

 see that as regards Ampere, Ohm's law was not far off. But in 

 the case of all his contemporaries this would be very far from 

 true, however much they concerned themselves, together 

 with Ampere, with Oersted's discovery; this is seen in the 

 want of comprehension which they exhibited even seven 

 and more years later, when confronted with Ohm's achieve- 

 ment. 



Ampere's great step forward, taken in the year of Oersted's 



^ We may ask why not 'voltameter'? For it was not Galvani but 

 Volta who was the first to render possible and produce the subject of 

 measurement, namely steady electric currents, although certainly as a 

 continuation of Galvani's investigation. I can find no other reason than 

 perhaps the fact that Volta was still alive then, and Ampere wished to 

 honour the dead. As a matter of fact the word 'voltameter' was intro- 

 duced later by Faraday, for apparatus measuring current by its chemical 

 effects, after Volta 's death. 



