APPARATUS 9 



are connected with screws placed on the frame of the 

 coil in front. The secondary coil is usually arranged to 

 slide over and away from the primary, and there is a 

 scale in centimetres on one side of the framework on 

 which the distance of the secondary from the primary 

 coil can be read off. In some forms there is also a scale 

 on the opposite side graduated to show the relative 

 strengths of the induced current at different positions 

 of the secondary coil. A bundle of iron wires is placed 

 in the central space of the primary coil, the wires are 

 magnetised when a current passes iiAo the coil and thus 

 the induced current reaches its maximum more slowly 

 and is of longer duration. 



a. Single induction currents. Connect the poles A, 

 B of the cell with the screws p^, p 2 ' of the primary coil, 

 interposing an in-circuit key. The key should be left 

 open except when current is required. Connect the 

 secondary coil with screws 1 and 2 (Fig. 26) of the 

 short-circuit key, and the electrodes with the screws 1', 

 2'. A diagram of the arrangement is given in Fig. 4. 



Ace i.c.k P S s.c.k E 



Fig. 4. Ace., accumulator; i.c.k., in-circuit key; P, primary coil; S, 

 secondary coil; s.c.k., short-circuit key; E, electrodes. Note that 

 closing the in-circuit key causes an induced current in the secondary 

 coil, and closing the short-circuit key prevents the current from 

 passing into the electrodes. 



An in-circuit key (open) must be used in the primary 

 circuit, partly to avoid waste of current, but more especi- 

 ally because after a certain limited time an accumu- 

 lator rapidly deteriorates if it is not re-charged. With a 



