58 
MESSRS. G. F. C. SEARLE AND T. G. BEDFORD 
the sectors d and e are connected bv the resistance coils D, D, while the sector c is 
insulated. The spaces between the sectors, about 1'5 millinis. wide, are filled with 
ebonite so that the springs pass smoothly over them. The primary circuit PC joins 
the sectors «, e. The two stops 5, id'' arrest the motion of the arm when I has got well 
on to either a or e, so as to be clear of h or d. To avoid any possible break in passing 
from one sector to the next, the spring I is made in two portions, /o, each of which 
presses upon the sectors. The spring k is arranged so as to touch the sectors only 
at its ends, /jj, The lengths of the sectors and of the springs are arranged so that 
the key performs the reversal in the following manner :—At the start I is on a, and 
h is on e ; both parts of I reach h before k reaches a. While I is on 5 the primary 
current falls from E/(II + T) towards the limit E/(T1 + T + D), where E is the 
voltage of the battery, T the resistance of the battery and ampere-meter, and Id the 
resistance of the rest of the circuit. Before I leaves b, k-^ reaches a, and then there is 
no E.M.F. acting round the primary circuit except what arises from the very small 
Internal resistance of the key, and then also the resistance offered to any current 
flowing in the primary circuit is reduced from R + T + D to R. When I is on c, 
one pole of the battery is insulated, and there is no E.M.F. at all acting on the 
primary circuit, whose resistance remains R. When I reaches d, and A is still on e, 
a very small E.M F., due to the internal resistance of the key, acts in the reversed 
direction on the primary circuit. The resistance offered to a current in the primary 
circuit is still R. While I is on d and before it reaches e, k.^ leaves e ; the resistance 
of the circuit is now raised to R + T + D, and the voltage E is introduced. When 
/ reaches e the resistance is diminished to R + T, the voltage remaining E. 
The resistances D need not be large; they are only used at all in order tt) save the 
battery and the ampere-meter. If the resistances D were made very small, and if T 
were small also, a large current Avould flow through the ampere-meter when 1 is on h, 
on o, and k.^ on e, causing damage to the meter. The resistances D need oidy be 
great enough to prevent the ampere-meter from being damaged l)y too strong a 
current. 
We have only had opportunity to use this key in a few experiments, but as far as 
we have tested it we are satisfied with its action. The motion of the spot of light on 
the scale of the dynamometer is now quite free from the sudden jumps which it 
exhil3ited when the key of § 31 was used. 
The Choking Coil. 
§ 33. The “choking coil,” which was inserted in the })rimary circuit to introduce 
great self-induction, has a core built up of 149 armature rings of an average thickness 
•0714 centim., the inner and outer radii being 8‘5 and 11'7 centims. respectively. 
The mean circumference is thus G3’7 centims. and the cro.ss-section 34‘0 sq. centims. 
11 the core had been solid and not laminated, the eddy currents induced in it would 
* By ;ui ovei'«ighL the wire from // to WA has been drawn through 
