ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
155 
spending value of U, viz., 17780, we obtain W = 7890. Thus, when the proper 
corrections are applied, the method yields approximately correct results even 
with an ordinary key and without a choking coil. It was a little disheartening to 
find so great a discrepancy betw^een W and U as the numbers 7409 and 17800 
indicated, and we were glad to find five years later that the discrepancy is 
satisfactorily accounted for by the more comjDlete theory. 
(h.) Using the key of § 31 as well as the choking coil, we found (August 8, 1899) 
for a soft iron wire '0324 sq. centim. in section, with Hq = 18'75, Bq = 15800, 
W = 19400, after correction for damping and for finite arcs, 
S = 59'5 ohms, -j- ^3 = 34'01 centims., S (dj + ^ 3 ) = 2024, 
S = 118‘5 ohms, -f* ^3 = 17'08 centims,, S {0^ + = 2025. 
For the secondary coil n = 1285, I = 11’5 ; also Q = l/87r, a = 10““, and thus, 
when S = 59'5 ohms, X/Y<QS//nV < 1‘66. But the effect of Y, the correction 
arising from the conductivity of the secondary circuit, in causing a change in the 
product S + do), is quite insensible, and hence the eddy current effect, X, is 
negligible. In this case, as closely as we could measure, U = W, 
(c.) With the same key and choking coil we found (August 7, 1900) for a bundle 
of ten pianoforte-steel wires, of total section ‘0255 sq. centim., with Hq= 35’7’ 
Bo = 16460, W = 75200, 
S = 39'2 ohms, -|- ^3 = 22'16 centims., S (d^ + dg) = 869, 
S = 78'8 ohms, d^ + d^ = 11‘04 centims., S (d^ + do) = 870. 
Here n = 600, I = 9‘6, so that when S = 39‘2 ohms X/Y < QS//10?^^c^ < '416. Here 
we have divided by 10, since the specimen is formed of ten wires (§ 9). In this case 
X and Y are l)oth negligible. 
id.) To illustrate tlie method of finding the correction by doubling the resistance, 
we take a test made on a bundle of ten iron wires coated with shellac varnish to 
prevent eddy currents from wire to wire. The total section was '0412 sq. centim. 
With the key of § 31, but with an inefficient choking coil, we found (August 9, 1900), 
when H = 35’8, Bq = 15550, 
S = 58‘6 ohms, d^ + d^ = 6‘68 centims., S (d^ + = 391, 
S = 118'0 ohms, d^ + dg = 3‘21 centims., S (d^ + dg) = 379. 
The difference between the products is 12, and thus the correction to be subtracted 
from 391 is 24. We had n = 1285, I — 11'5 centims., so that since there are ten wires, 
when S = 58'6 ohms, X/Y < T62. But we make only a small error in taking X/Y 
= ’162, and thus the total correction to be subtracted from 391 is 24 X 1’162 or 28. 
Hence the value of S (d, + dg) to be used in finding U is 363. 
VOL. cxcviii.—A. 
K 
