50 Measurement of the Magnetic Properties of Iron. [May 24, 



vol. 184, A, pp. 531 542. The results now given have been to a 

 large extent obtained by the same method, namely, from the curves 

 giving the relation of the current flowing in the circuit to the time 

 measured from the application or the reversal of the impressed 

 E.M.F. on the circuit. In this case, however, the personal element 

 has been eliminated from the curves by the application of the 

 autographic recorder referred to as under construction in the 

 previous paper. This apparatus, which is a modification of the 

 " Thomson siphon-recorder," has been found to work satisfactorily, and 

 has considerably increased the ease and the accuracy with which the 

 curves can be produced. A description of the apparatus and 

 specimens of the curves drawn by it are included in the paper. 

 There is also included in this paper a description of the apparatus 

 and method of experiment in the application of a wattmeter to the 

 determination of the energy dissipated by transformers under 

 E.M.F.'s of different frequency of alternation. The accuracy of the 

 measurements so made were checked by comparison with the results 

 of measurements made by Joubert's instantaneous contact method. 

 The apparatus and method of experiment adopted for the application 

 of this method were to some extent different from those commonly 

 employed, and they are therefore described. 



The results of some further experiments on the large electromagnet 

 used in the previous experiments, and described in the paper above 

 referred to, are given, but a large part of the results quoted in this paper 

 refer to closed circuit transformers of the types manufactured by the 

 Westinghouse and the General Electric Companies. The experi- 

 ments have been chiefly directed to the following points : 



1. A Comparison of the Total Energy required to produce Different 

 Magnetic Inductions, and the Corresponding Dissipation of Energy. 

 In connection with this, the effect of air gap in the magnetic circuit 

 has been investigated somewhat more fully. It is shown that, by 

 introducing a moderate air gap, the energy dissipated for a given 

 induction through the coils may be reduced one-third. 



2. The Law of Variation of Hysteresis with Variation of Induction. 

 The experiments indicate that, although for any special case the 

 energy dissipated can be approximately expressed by an equation of 

 the form E = AB a , that both A and a. are different for different kinds 

 of iron. It seems probable, also, from the results obtained, that a. is 

 not absolutely constant for any one iron, but that it increases with 

 increase of B. 



3. The Effect of Increased Frequency of Cyclic Variation of Mag- 

 netism on the Dissipation of Energy. In this investigation a trans- 

 former, the iron case of which was made up of very thin sheets, was 

 used. The thickness of the sheets was about 16-100ths of a millimetre, 

 and the sheets were insulated from each other by means of thin 



