Oscillation, Capacity and Inductance

Radio Record, Volume III, Issue 28, 24 January 1930, Page 30

Oscillation, Capacity and Inductance

rd Explanation. by Sir

Oliver

Lodge

I SUPPOSE everyone knows that a circuit which includes both inductance and capacity is liable to oscillate, at a certain definite frequency, dependent on the produce of the inductance ‘and capacity (writes Sir Oliver Lodge in "Popular Wireless"). If you double one and halve the other, the rate of oscillation remains the same, If, on the other hand, you halve both, the rate of oscillation would be quadrupled: it would give what in sound is equivalent to a double-octave rise in pitch. Increasing either the capacity or the inductance lowers the rate of vibration. By employing a very big condenser and a coil with a great. number of turns, one can reduce the rate of vibration actually down to acoustic frequency. Electric rates of oscillation are usually extremely rapid, a hundred-thousand a second, for instance. But with a very big coil and a very big inductance, the rate of oscillation can be reduced to a oc ee we second, if it is made to disturb the air, will give a musical note, Whistling Sparks. An experiment of that kind is best performed with a Leyden jar as the capacity, or with some other strong condenser that can stand a big potential, so that it will give a spark. A spark is a means of disturbing the air: it heats the air, and usually gives a sort of crack, which is quite audible. But if the spark is taken from an oscillating circuit, one that vibrates about six thousand or two thousand a second, you won’t hear a crack, you Will hear a whistle, or ‘even a musical tone of lower pitch than that. Last century I used to demonstrate these whistling sparks, with a battery

of ‘Leyden jars as the capacity, and with a great coil of wire as the inductance,’ One could thus bring the spark-note down till it corresponded with some df the notes on a piano. In some engineering undertakings these electric. oscillations or surgings occasionally gave trouble. When. Dr. Ferranti, long ago, laid large electric underground mains from Deptford to London, the lines had'a‘great capacity ; there was also a considerable amount of inductance in _ big dynamos employed. So the result was that the circuit had a natural rate of vibration, rather slow but decidedly powerful; and accordingly the voltage rose above what was provided for, and was liable to burst the insulation, until proper precautions were taken 'to combat the tendency. If you have a long bath half filled with water, and tip the bath lengthways, the water oscillates to and fro, and may easily splash over the edge. That sort of thing happened in the electrical case. The surging electricity overflowed or splashed over or burst through the insulation; or, if it didn’t, there was a danger of its doing so. This was very instructive, and excited a good deal ‘of interest. The same sort of thing would be less likely to occur in a land line, because a land line has much less capacity, and therefore the energy stored would be less. That is one reason why in longdistance electric power transmission engineers prefer overhead lines to underground mains! Any surgings that may be encountered ,are less likely to be violent. One can get surgings even in a land line. I used to stretch long copper ‘wires-if you can call them wires-as thick as one’s finger, round a big theatre, excite high voltage surgings by means of Leyden jars, and show that the sparks that could be obtained from the long leads were very much longer than corresponded with the applied voltage. | Electric Momentum. If the natural length of spark wa # inch the surgings might give a as much as‘’3 inches by the accumulated momentum. For it must be understood that the magnetic field which surrounds a current confers momentum upon it; and when this is combined with the elastic storage of energy called capacity, it behaves like a loaded spring which can oscillate to and fro with violence, the rate of oscillation depend- ing on the elasticity and the load. The load or the. inertia corresponds to in‘ductance in the electrical case, the elasticity corresponds to capacity. I have seen the effect.of these electric surgings in some of the early largescale radio aerials, when a great steel

mast formed an appendage to the circuit. An’ electric current.surged up and down in this mast with such violence that blocks of timber . placed against the mast to'strengthen it in a storm were charred and smoked, and might have caught fire. : This was notably the case in, 2 ee installation arranged by Goidschmidt’ near Hanover ; and very strong insulators had to be in order to prevent their destruction. Wireless operators, as a rule, are not concerned with these violent effects; the self-generated oscillations at a receivying station only produce howling. But the fundamental principles ate much the same, whether great or small power is employed; and the rate of oscillation can be reckoned in the same sort of way.