JOSEPH HENRY 127 



stop the motion, the current stops also. Now take the mag- 

 net out and the current again flows, but in the opposite direc- 

 tion. 



Here we have the principles on which the modern dynamo is 

 constructed, by which electric roads are now run. Unfortunately 

 there were very few scientific societies and scientific men in this 

 country; and Henry himself had no idea what an epoch-making 

 discovery this was; so he did not publish it immediately, but 

 went on trying to perfect it before describing it in print. While 

 he was doing this he found that Faraday had made the same 

 discovery in England, and published it to the admiring scientific 

 world. It was a remarkable illustration of Henry's high charac- 

 ter that he never complained of not receiving the credit of having 

 been another discoverer, but subsequently spoke of "Faraday's 

 admirable discovery" as if it was something with which he had 

 nothing to do. C*~V$WA^ tiu flAM^tr 



Another discovery which Henry was the first to publish, and 

 for which he has entire credit, is that which is known as the self- 

 induction of an electric current. Under certain circumstances 

 when a long current is suddenly broken there is a momentary 

 flash in the opposite direction, and the longer the wire through 

 which the current is passing the stronger is this flash. This is the 

 cause of the bright flashes that are so often seen at night on the 

 trolley of an electric car as it is running along the wire. The 

 trolley makes a slight jump; the current is thus broken, and the 

 self -induced current jumps across the space with the brilliant flash 

 which we all must have so often noticed. 



Another of Henry's discoveries and one of a very curious char- 

 acter was that, when a flash of electricity suddenly passes through 

 a wire when a Leyden jar is discharged, for example what takes 

 place is not a single passage of electricity, but a vibrating of elec- 

 tricity back and forth through the wire. These vibrations are so 

 rapid that they all take place in a much smaller time than the 

 human faculties could ever appreciate, perhaps the ten thousandth 

 or one hundred thousandth of a second, perhaps I ought to say the 

 fraction of a millionth of a second. The question may arise how 



