Induction-Coil Potentials Page 577

Final Experiments and Conclusions.

One of the experimental curves for each case is shown in Plate VIII. figs. 7, 8, 9^*. In obtaining them the external circuit (i.e. the battery branch) was made as nearly non-inductive as possible, the connecting wires being well twisted together and the current being varied by changing the number of battery cells, rather than by inserting resistance coils. This was necessary because the primary condenser was connected directly across the coil. If a large battery and rheostat is used there is much sparking at the interruptor, and the maximum secondary potential is small and not even approximately proportional to the primary current. Two forms of interruptor were used, one consisting of a steel rod making contact with an iron plate, the other of an amalgamated copper rod dipping into mercury, in each case under paraffin oil. Even when the external circuit was nearly non-inductive there was generally sparking at the interruptor with the stronger currents, but occasionally a sparkless interruption occurred, accompanied by an exceptionally high potential in the secondary coil. The amplitude of the electrometer curve

^* The three photographs shown were obtained on different occasions, on which the electrometer had different degrees of sensitiveness.