504 Measurement of the Magnetic Properties of Iron. [May 19, 



maximum time. Thus for the circuit under consideration, and with 

 successive repetitions of the current in the same direction, it takes 

 longer time for the current produced by an impressed E.M.I 1 , of 

 4 volts to reach 95 per cent, of its maximum than it takes for the 

 current produced by either 3 or 5 volts to reach 95 per cent, of their 

 maximum. The results show also that, within considerable limits, 

 the time required for the current to become uniform is on the whole 

 nearly inversely proportional to the impressed E.M.F., and that for 

 moderate values of the E.M.F. the time may be very great ; when the 

 E.M.F. was 2 volts, and the current sent in such a direction as to 

 reverse the magnetism left in the magnet by a previous current of 

 the same strength, the time required for the current to establish 

 itself was over three minutes. The difference of time required for 

 repetition and for reversal of previous magnetisation was also very 

 marked when the iron circuit was closed. The results show that- 

 great errors may arise by the use of ballistic methods of experiment, 

 especially when weak currents are used, and that for testing re- 

 sistances of circuits containing electromagnets, a saving of time may 

 be obtained by using a battery of considerable E.M.F. 



Another set of experiments gives the effect of successive reversals 

 of the impressed E.M.F. at sufficient intervals apart to allow the 

 magnetisation to be established in each direction before reversal 

 began. In this set also the effect of cutting out the battery and 

 leaving the magnet circuit closed is illustrated, showing that several 

 minutes may be required for the magnet to lose its magnetism by 

 dissipation of energy in the magnetising coil. The effect on these 

 cycles of leaving an air space in the iron circuit is also illustrated. 

 It is shown that a comparatively small air space nearly eliminates 

 the residual magnetism and diminishes considerably the rate of 

 variation of the coefficient of induction and the dissipation of energy 

 in the magnet. 



Several cycles are shown for the magnet used as a transformer 

 with different loads on the secondary. The results give evidence 

 that there is less energy dissipated in the iron the greater the load 

 on the secondary of the transformer. 



Some experiments are also quoted which go to show that the 

 dissipation of energy due to magnetic retentiveness (magnetic 

 hysteresis) is simply proportional to the total induction produced 

 when the measurements are made by kinetic methods. Reference is 

 made to the recent experiments of Alexander Siemens and others 

 which seem to confirm this view. 



