AGNEW: CURRENT TRANSFORMER 53 



The solution saturated with mercuric iodide at 20° and at the 

 density of ordinary orthorhombic sulphur (2.07) has, for concen- 

 trations between 2.25 and 3.4, the relation d = 5.39 n - 6.0865, 

 where d is the density and n the refractive index. 



ELECTRICITY. — A study of the current transformer with par- 

 ticular reference to iron loss. P. G. Agnew. Communicated 

 by E. B. Rosa. To appear in the Bulletin of the Bureau of 

 Standards. 



It has been generally assumed that the ratio of a current trans- 

 former always decreases with increasing current, but examples 

 can be given in which the ratio curve slopes in the opposite direc- 

 tion, or even passes through a maximum. The ratio and phase 

 angle performance may be accurately computed from the magnetic 

 data of the core. In nearly all cases the slope of the ratio curve 

 may be qualitatively predicted from the value of the Steinmetz 

 exponent in the equation W = kB where W = total iron loss, 

 B = max. flux, k and c are constants. But the iron losses, par- 

 ticularly at the low flux densities used, depart too widely from 

 such a simple law for accurate work. 



The slope of the ratio curve may be accurately computed from 

 the slope of the curve obtained by plotting the core loss against 

 the flux on logarithmic paper. It is proposed that this logarith- 

 mic slope, or logarithmic derivative shall be called the "ratio of 

 variation." It is much more useful than an actual exponent. 

 The methods now in use for determining the "exponent" fail to 

 give a true exponent that will satisfy an equation of the form 

 W = kB unless z is a constant. The quantity actually deter- 

 mined by these methods is the ratio of variation. 



The wave form of the secondary current of a transformer may 

 be considered to be the same as that of the primary current for 

 even the most precise measurements, as the distortion within the 

 transformer is entirely negligible, as may be shown experimentally. 

 For a good transformer with sinusoidal primary this distortion 

 amounts to less than a part in a million in terms of effective 

 values. While the effect of ordinary variations in wave form on 

 ratio and phase angle may be detected by accurate measurements, 



