540 
MR. T. GRAY 0^4" THE MEASUREMENT OF 
It may be remarked that tlie curve of secondary current is slightly affected by the 
heating of the lamps, which became a dull red towards the end of each reversal of 
current in the primary. The effect is slight, however, and the equality of the back 
E.M.F. on the primary, and the forward E.M.F. on the secondary, is as perfect as the 
experiment can show. The heavy dot-dash line encloses an area which represents the 
total dissipation of energy in the iron and in the secondary coil and circuit. AYe have 
here the means of illustrating the effect of Foucault currents on the L curve. It will 
be noticed, by comparing figs. 17 and 18, that the high values of the L curve are 
displaced to right and left by the existence of the secondary currents, which, by their 
demagnetizing action, allowmd the primary current to become stronger before taking 
the inflection due to high permeability. The magnetizing force being now the differ¬ 
ence of the magnetizing forces of the twm coils, the curve of coefficient of induction 
should be plotted to this difference as abscissae. This is done in figs. 20 and 21, which 
show that the maximum value of the coefficient comes earlier the stronger the current 
in the secondary. Fig. 19 is the same as fig. J 8, except that the resistance of the 
secondary is now simply that of its own coil and the galvanometer circuit, namely, 
5’62 ohms. There is now, of course, a greater dissipation area and a greater displace¬ 
ment of the L curve. There are one or two points of interest in connection with the 
L curves and the curves shown by fine dotted lines which may be noted. The L 
curve, when calculated from the primary current, besides being displaced, has a higher 
maximum value the greater the current in the secondary. The dissipation of energy 
in the magnet itself is less the greater the dissipation in the secondary circuit. When 
there was no current in the secondary, the energy dissipated per cycle was 1950 X 10® 
ergs ; when the resistance in the secondary W’as J 27 ohms, this fell to 1440 X 10® ergs, 
and when the secondary was short-circuited, it fell to 946 X 10®, or less than half the 
first value. When the differences of the magnetizing forces are considered, the L 
curve has its maximum nearer to zero current, the greater current in the secondary, 
and has nearly a constant value. 
The effect of varying the impi-essed E.M.F. on the dissipation of energy is 
illustrated in figs. 22 to 25, the abscissm of vdiich are proportional to the impressed 
E.M.F., and the ordinates to the induction as calculated from the reversal of current 
curves which were similar to that shown in fig, 12. Although the number of 
experiments here shown are not sufficient to allow any law^ to be accurately’ deduced 
from them, the results are sliown by the black dots in the curves figs. 26 and 27. 
Fig. 26 is drawn from an equation of the form e = AE + BI, wdiere e is the energy 
dissipated, E the impressed E.M.F., I the total induction, A and B constants. For this 
particular curve A = | and B = 25300. The curve shown in fig. 27 has impressed 
E.M.F"'. for abscissae and dissipation for ordinates, and is drawn to show that for the 
range of experiment taken the dissipation was practically proportional to the 
mqDressed E.M.F. Indej^endent experiments indicate that the amount of energy’ 
dissipated in the secondary circuit when the magnet is used as a transformer, in the 
