56 
MESSRS. G. F. C. SEAELE AND T. G. BEDFORD 
inductive coil D of 100 ohm.s being put in parallel with the primary circuit, PC, as in 
fig. 3. When the self-induction of the primary circuit is large the current does not 
stop very rapidly when contact is broken, for the primary current can still flow on 
through D, and thus the decay of the current is much less raj^id than when it has to 
overcome the high resistance of the spark, as in § 28. When contact is re-established 
the current rises in exactly the same manner as with the simple rocking key. The 
practical objection to the method is that it does not allow of the current being 
directly measured by the convenient Weston ampere-meter, for the current must always 
flow in the same directioji through this instrument. A Kelvin graded galvanometer 
or a shunted mirror galvanometer, MG, was accordingly used with this kev. 
§ 31. The fourth reversing key used is shown in fig. 4. The batteiy is con¬ 
nected through the Weston ampere-meter WA with the terminals e, f. on an 
ebonite arm working al)Out a ])ivot at O. The terminals e and f are connected to two 
brass springs, one at each end of the arm. One spring slides over a series of studs, 
while tlie other slides over two Ijrass sectors c, d. Half the .studs are connected 
to the teiininal a by resistance coils in the manner slmwii in fig. 4, the remaining 
studs being connected to the terminal l>. The sectors d. c. are connected to the 
terminals a, h ; the ])rlmarv circuit, PC, joins the key at a and h. The resistance 
coils which connect the studs have I'esistances of 40 and 200 ohms. The ' two 
stops s, s serve to limit the motion of the arm, so that in its extreme jiositions 
the spring connected witli e presses u|)on tlie .stud nearest to either a or 6. The 
Avidth of the springs is suflicient to ensure that before tlie S 2 )ring leaves one stud it 
is in contact Avith the next one. 
When the arm is in either of its extreme positions the total resistance of the circuit 
is Pv + T, AA'here T is the resistance of the battery and ampere-meter and Pi is the 
resistance of the rest of the circuit. When the spring connected Avith e comes on to 
the next stud the resi.stance is 11 fl- T -j- 40, and one more step makes the resistance 
P + T + 240. When the spring conies to the next stud the E.M.F. acting on the 
primary circuit is re\’ei'sed. 
