200 Transactions, — Miscclhmeous. 



Cui've 10 shows what a marked difference there is in the 

 current-curve in the secondary when iron is in the circuit. 



A secondary was wound over the hiininated core of a small 

 induction-coil, and the duration of the secondary current 

 determined. The current-curve exhibits two maxima, and is 

 far more irregular than curve 9 (B). This was first thought 

 to be due to some experimental error, but farther investiga- 

 tions showed that the same peculiarity was exhibited by all 

 the curves obtained. Curves were also obtained w'hen finely- 

 laminated n'on and steel rings vvere used. The duration of 

 the secondary could be varied by altering the resistance. 

 When a large resistance was placed in the secondary the 

 duration was very short. In the cases above considered the 

 induced currents lasted about ^^^q of a second. 



When solid iron is in the current the duration of the 

 secondary is greatly prolonged, and is independent in a great 

 measure of the rcaiataiice of the secondary. 



X solid iron ring was taken and wound with appropriate 

 magnetizing and ballistic coils. It was found that the 

 secondary was of long duration. When 1,000 ohms were 

 added to the current very little difference was observed. 



If the lines of force had passed suddenly out of the 

 primary, as in the case of the laminated core, the duration of 

 the secondary induced current would have been diminished 

 by increasing the resistance in the secondary ; and yet in the 

 case of the iron ring the effect was scarcely appreciable. 



Clearly, then, the lines of force must pass out of the 

 primary very slowly to account for the observed effect. The 

 magnetic force in the iron changes very slowly when the 

 current is broken, on account of the induced currents in the 

 mass of the metal tending to prevent the decay of magnetic 

 force through the iron. 



Decay of Magnetic Fokce in Iron and Copper Cylinders. 



The very slow rate of decay of the magnetic force in an 

 iron cylinder, which was observed in the experiments on the 

 induced current at break, led to a series of more detailed 

 experiments on the rate of decay of magnetic force when a 

 uniform field was suddenly removed. 



The subject is treated mathematically, p. 352-358, in 

 Thomson's " Recent Researches," but I am not aware that 

 the subject has been experimentally verified. 



Suppose a metal cylinder be placed in a solenoid, and a 

 steady current be sent round the solenoid. If the current is 

 suddenly broken there are induced currents in the mass of 

 the metal tending to maintain the original state of the mag- 

 netic field, and instead of sinking abruptly the field decays 

 very slowly. 



