458 Sir Oliver J. Lodge [Feb. 28, 



to possess inertia ? Faraday discovered that an electric current had 

 a property which bore some analogy to inertia, a property clearly 

 depending on its magnetic field. Every current, even a convection 

 current, is necessarily surrounded by lines of magnetic force, and 

 when the magnetic field is intense the current behaves as if it had 

 considerable inertia. Faraday at first called the effect " the extra 

 current." Maxwell called it "self-induction." The latter is the 

 better name. 



To show it I start a current in a circuit containing a stout ring 

 of laterally subdivided iron round which the current-conveying 

 wire is wound, and I put in circuit an instrument which only 

 responds when the current has risen to nearly its full strength. A 

 current usually rises, what is called, instantaneously, but here there is 

 a very noticeable delay between pressing down the key and the 

 response of the instrument. [Experiment.] The lag shown is only 

 a second or two, but with care I can adjust it till it is a quarter of a 

 minute. Such delay or lag in establishing a current would be fatal to 

 electric telegraphy. In practice the delay is reduced to a minimum, 

 by using its early values, and the actual response is exceedingly quick. 

 Still, the law of rise of current is quite definite, there is no exception, 

 it is only a question of degree ; and the law is the same as that 

 appropriate to the pulling of a barge on a canal. A barge gets up 

 speed slowly, at a rate depending on its mass or inertia, and it 

 ultimately attains a steady speed when the resistance balances the pull. 



That is exactly the case of a steady current obeying Ohm's law, 

 the E.M.F. is balanced by the resistance, the propelling force is zero, 

 and the current flows by what we may call its own inertia — its own 

 momentum. 



To stop the current you must either increase the resistance 

 or suspend the propelling force. If you interpose an obstacle 

 suddenly, the motion stops with violence — a collision in the case of 

 a train or barge, a flash in the case of electric current. This is 

 what Faraday called " the extra current at break," and if you are 

 holding the wires in your hand when a current is suddenly broken in 

 a circuit of large self-induction you may get a nasty shock. 



If you could abolish electric resistance a current would go on for 

 ever without propelling force. 



An amazing experiment has been made by Kamerlingh Onnes at 

 Leiden, who first cooled a metal ring down to within four degrees of 

 absolute zero by means of liquid heUum, and then started a current 

 through it by a momentary magnetic impulse. Instead of stopping 

 in a minute fraction of a second, as usual, the current went on and 

 on, not for seconds but for days. In four days it had fallen to half 

 strength, and there were traces of it a week later. A most suggestive 

 experiment as to the nature of metallic conduction, as well as a 

 demonstration of the fly-wheel-like momentum of an electric current ! 



This electromagnetic analogue to mechanical momentum or 



