46 
MESSRS. G. F. C. SEARLE AND T. G. BEDFORD 
precautions to secure that dCjclt should never be large, it may give quite inaccurate 
results for soft iron unless the j)roper precautions are taken, A considerable jDortion 
of the time sjDent upon this research was occupied in investigating the precautions 
necessary to secure accuracy. 
§ 11, If we consider the effect of changing the area of section of the iron we shall 
see that the jDart of the correction due to the conductivity of the secondary circuit 
increases as the section diminishes. For neglecting L and M, (14) becomes 
dt 
. . (16). 
Now for a given kind of iron, if we wish to obtain the same throw of the 
dynamometer, when we halve A we must double n. Thus we must regard An as 
determined by the sensitiveness of the dynamometer. Hence the of 
correction which depends iq^on S is proportional to n, and therefore inversely propor¬ 
tional to A. This explains the difficulty we met with in the earlier stages of the 
work. We were able to get accurate results for specimens built up of fine wires, with 
a total section of 1 sq. centim., though we failed to do so for specimens with a section 
of 1 or 2 sq. millims. 
The part of the correcting term in (16) depending ujDon S is inversely proj)ortional 
to /. The circumference of the ring should therefore be large. When, as in most of 
our experiments, a straight wire of finite length is used, the secondary coil should be 
wound upon a long bobbin and not be heaped up at one part of the wire. 
§ 12. Effect due to Air alone. —If there l>e no iron in the system B = H everywhere, 
and W = 0 ; if, further, there be no metal, e.g., brass, inside either the primary or the 
secondary coil X = 0 also. In this case (14) reduces to 
CT 
(di + 6 ^ — 
1/BrtdA 
s\ " 
(477nAN -f M) 
(17). 
Since dt is necessarily positive, dQjdt has the same sign as c/C, and hence the integral 
cannot vanish. There will, therefore, always be a small throw of the dynamometer 
even when there is no metal in the coils. But since c/B/c/H is always positive and is 
generally large, it is easily seen that unless L and M (§8) are very large in 
comparison with the quantities to wliich they are added in (17), the throw, when the 
iron is out, is small compared with the part of the throw which is due to the correct¬ 
ing integral in (14), when the iron is in. Tliis deduction we verified by experiment, 
for in many trials we never found any case in which we could detect any throw when 
there was no metal in the magnetising coil. 
Determination of the Correction for the Finite Conductivity of the Secondary 
Circiiit, and of the Energy dissipated hy Eddy Currents. 
§ 13. It is convenient to have a single symbol for the quantity NC'P(di + 6f)/An(f), 
and we shall use U for this purpose. Thus U is the value of the hysteresis loss 
