j-J MESSRS. G. F. C. SEARLE AND T. G. BEDFORD 
For a single wire 47 ceiitims. long and '00412 sq. centim. in section, placed in a 
solenoid 60 centims. long, we had (G—A)/A = 37, and found (§ 79), 
|/u/H = *118 X 37 = 4-36. 
The limits of B were dr. 9850, of H d: 10‘67, and of h dr '0084. 
§ 20. Tliese corrections are also applicable to tlie area of a cyclic B —H curve 
drawn by the aid of a ballistic galvanometer. With respect to the maximum 
induction B^, when the current C,j is reversed, the throw of the galvanometer 
measures ABq -f (G—A) Hq, where it will l)e sufficient to take Hq as 47rN'C,„ since 
(G—A) Hq is small com])ared witli AB^. But in finding the maximum magnetic 
force Hq, tlie correction may not he negligible, and must be subtracted from the 
(quantity 477 N'Cq. The value of //q corresponding to Hg is easily found l)v the method 
described in Appendix II. 
|HdB = 102100 , |/idB = 72'3 
G - A 
' A 
Description of the Apparatcs. 
§ 21. We now jiass on to describe the instruments employed in the experiments. 
It will perhaps conduce to clearness if we leave till last the description of the special 
appliances which were designed in order to secure accuracy. 
IVie Ampere-Meter. 
§ 22. In all except the preliminary part of the work a Weston direct reading 
ampere-meter was used for the measurement of the primary current. The instrument 
only read up to 1'5 amperes, hut when the largest number of windings on the 
solenoid (§ 24) was employed this current gave a magnetic force of about 108 C.G.S. 
units. Tlie instrument reads in one direction only, and tlius it is necessary to join 
one of its terminals directly to one pole of the battery, so that the current always 
flows tlirouo'h it in the same direction. 
The Eartlh Inductor. 
§ 23. The earth inductor, whicli served throughout our experiments to produce a 
standard change of induction, was a simple coil of 100 turns, having a mean radius 
of about 18'2 centims.; its resistance is 1'345 ohms. Using a f4ark cell as a 
standard of E.M.F., the change in the number of linkages of lines of induction 
with the circuit when the coil is turned over through 180° about a horizontal axis 
was found to be 8'72 X 10^ C.G.S. units. 
We prepared a resistance coil of copper wire with the same resistance as the earth 
inductor. We could thus substitute this coil for the earth inductor when we desired, 
for the reason given in § 9, to get rid of the self-induction of the earth inductor and 
