Induction-Coil Potentials Page 582

the best effects would be obtained without any condenser. The above theory, however, points to a maximum spark-length with a certain condenser, at any rate in the case of negligible resistances, even with a perfectly sudden interruption.

The following experiments were therefore undertaken with a view to testing this point by observing the lengths of spark obtained with various capacities and different forms of interruptor. The condenser used was a subdivided mica microfarad, in which the fractions could be connected either in parallel or in cascade. This was in each case connected directly across the interruptor. The electrometer, though not used in these experiments, was left connected to the secondary coil in parallel with the ball electrodes, so that \(L_2 C_2\) and \(k^2\) had the same values as in the experiments described above.

Experimenting first with the mercury interruptor and one secondary cell (\(i_0\) about 1·1 amperes), no amperemeter or rheostat being included in the circuit, it was found that the mica condenser was as a rule sufficient to prevent any visible spark at the interruptor, even when all its sections were in cascade. There was occasionally a spark at the interruptor but no account was taken of the result when this occurred. The capacities were taken in pairs, each two being tried alternately many times at the same distance. Thus ·5 mfd. was found to give a longer spark than 1 mfd., ·3 longer than ·5. The longest spark was given by ·05 mfd. Special attention was given to this capacity and ·025 mfd., which were tried on many occasions. A distance could always be found at which ·05 gave a spark every time, while ·025 always failed. The distance was not the same on all occasions but was generally about 19·5 mm., and the difference of the sparking distances for these two capacities was about 1·5 mm. A longer spark was given by ·05 than by ·1 mfd., while ·025 and ·02 did not show any appreciable difference. A smaller capacity, in the form of an oil condenser of about ·002 mfd., was tried, but in this case there was always sparking at the interruptor.

With the mercury break and one cell, therefore, the most effective capacity was about ·05 mfd.^*, but there were indications of a second (smaller) maximum at about ·3 mfd. this capacity giving consistently a rather longer spark than ·2 mfd.

When tried with two cells, an amperemeter and a rheostat

^* The same result was found with a motor interruptor of the centrifugal type.