H 



NATURE 



[May 7, 1891 



municating motion to the air after a very few vibrations. 

 The case of a Leyden jar discharge is more like the case 

 of a mass on a spring than the case of a pendulum, 

 because in the cases of the Leyden jar there is nothing 

 quite analogous to the way in which the earth pulls the 

 pendulum : it is the elasticity of the ether that causes the 

 electric currents in the Leyden jar discharge, just as 

 it is the elasticity of the spring that causes the 

 motion of the matter attached to it in the case 

 of a mass vibrating on a spring. It is possible to 

 push this analogy still further. Under what conditions 

 would the spring vibrate most rapidly 1 When the spring 

 was stiff and the mass small. What is meant by a spring 

 being stiff.? When a considerable force only bends it a 

 little. This corresponds to a considerable electric force 

 only electrifying the Leyden jar coatings a little, i.e. to 

 the Leyden jar having a small capacity. We would con- 

 sequently expect that the discharge of a Leyden jar with 

 a small capacity would vibrate more rapidly than that of 

 one with a large capacity, and this is the case. In order 

 to make a Leyden jar of very small capacity we must 

 have small conducting surfaces as far apart as possible, 

 and two separate plates or knobs do very well. The 

 second condition for rapid vibration was that the mass 

 moved should be small. In the case of electric currents 

 what keeps the current running after the plates have 

 become discharged and recharges them again is the so- 

 called self-induction of the current. It would be well to 

 look upon it as magnetic energy stored up in the ether 

 around the current, but whatever view is taken of it, it 

 evidently corresponds to the mass moved, whose energy 

 keeps it moving after the spring is unbent and rebends 

 the spring again. Hence we may conclude that a small 

 self-induction will favour rapidity of oscillation, and this 

 is the case. To attain this we must make the distance 

 the current has to run from plate to plate as short as 

 possible. The smaller the plates and the shorter the 

 connecting wire the more rapid the vibrations ; in fact, 

 the rapidity of vibration is directly proportional to the 

 linear dimensions of the system, and for the most rapid 

 vibrations two spherical knobs, one charged positively 

 and the other negatively, and discharging directly from 

 one to the other, have been used. Hertz in his original 

 investigations used two plates about 40 cm. square, 

 forming parts of the same plane, and separated by an 

 interval of about 60 cm. Each plate was connected at 

 the centre of the edge next the other plate with a wire 

 about 30 cm. long, and terminating in a small brass knob. 

 These knobs were within 2 or 3 mm. of one another, so 

 that when one plate was charged positively and the other 

 negatively they discharged to one another in a spark 

 across this gap. An apparatus about this size would 

 produce waves 10 or 12 metres long, and its rate 

 of oscillation would be about 30 million per second. 

 As the vibration actually produced by these oscillators 

 seems to be very complex, the rate of oscillation can 

 only be described as " about " so and so. In a subsequent 

 investigation Hertz employed two elongated cylinders 

 about 15 cm. long and about 3 cm. in diameter, termin- 

 ated by knobs about 4 cm. in diameter, and discharging 

 directly into one another. Such an oscillator produces 

 waves from 60 to 70 cm. long, and, consequently, vibra- 

 tions at the rate of between 400 and 500 million per 

 second. Most other experimenters have used oscillators 

 about the same dimensions as Hertz's larger apparatus, 

 as the effects produced are more energetic ; but many 

 experiments, especially on refraction, require a smaller 

 wave to be dealt with, unless all the apparatus used be 

 on an enormous scale, such as could not be accommo- 

 dated in any ordinary laboratory. When we are thus 

 aiming at rapid rates of vibration, it must be recollected 

 that we cannot at the same time expect many vibrations 

 after each impulse. If we have a stiff spring with a small 

 weight arranged so as to give a lot of its energy to the 

 NO. I 123, VOL. 44] 



surrounding medium, we cannot expect to have very- 

 much energy to deal with, nor many vibrations, and, as a 

 matter of fact, we find that this is the case. The total 

 duration of a spark of even a large Leyden jar is very 

 small. Lord Rayleigh has recently illustrated this very 

 beautifully by his photographs of falling drops and break- 

 ing bubbles. We cannot reasonably expect each spark 

 to have more than from 10 to 20 effective oscillations, so 

 that, even in the case of the slower oscillator, the total 

 duration of the spark is not above a millionth of a second. 

 It is very remarkable that the incandescent air, heated to 

 incandescence by the spark, should cool as rapidly as it 

 does, but there is conclusive evidence that it remains 

 incandescent after the spark proper has ceased, and con- 

 sequently lasts incandescent longer than the millionth of 

 a second. What is seen as the white core of the spark 

 may not last longer than the electric discharge itself, and 

 certainly does not do so in the case of the comparatively 

 very slowly oscillating sparks that have been analyzed 

 into their component vibrations by photographing them 

 on a moving plate. The incandescent air remaining in 

 the path of such discharge is probably the conducting 

 path through which the oscillating current rushes back- 

 wards and forwards. Once the air gap has been broken 

 through, the character of the air gap as an opponent of 

 the passage of electricity is completely changed. Before 

 the air gap breaks down, it requires a considerable initial 

 difference of electric pressure to start a current. Once it 

 has been broken down, the electric current oscillates 

 backwards and forwards across the incandescent air gap 

 until the whole difference of electric pressure has sub- 

 sided, showing that the broken air gap has become a con- 

 ductor in which even the feeblest electric pressure is 

 able to produce an electric current. If this were not 

 so, Leyden jars would not be discharged by a single 

 spark. All this is quite in accordance with what 

 we know of air that is, or even has lately been, 

 incandescent : such air conducts under the feeblest electric 

 force. All this is most essential to the success of our 

 oscillator. Only for this valuable property of air, that it 

 gives way suddenly, and thenceforward offers but a feeble 

 opposition to the rapidly alternating discharge, it would 

 have been almost impossible to start these rapid oscilla- 

 tions. If we wish to start a tuning-fork vibrating, we 

 must g've it a sharp blow : it will not do to press its 

 prongs together and then let them go slowly : we must 

 apply a force which is short-lived in comparison with the 

 period of vibration of the fork. It is necessary, then, that 

 the air gap must break down in a time short compared 

 with the rate of oscillation of the discharge ; and when 

 this is required to be at the rate of 400 million per second, 

 it is evident how very remarkably suddenly the air gap 

 breaks down. From the experiments themselves it seems 

 as if any even minute roughnesses, dust, &c., on the dis- 

 charging surface, interfered with this rapidity of break- 

 down : it seems as if the points spluttered out electricity 

 and 'gradually broke down the air gap, for the vibrations 

 originated are very feeble unless the discharging surfaces 

 are kept highly polished : gilt brass knobs act admirably 

 if kept polished up every ten minutes or so. One of the 

 greatest desiderata in these experiments is some method 

 of making sure that all the sparks should have the same 

 character, and be aU good ones. 



{To be continued^ 



THE ROYAL SOCIETY SELECTED 

 CANDIDATES. 

 'T^HE following fifteen candidates were selected on 

 -*■ Thursday last (April 30), by the Council of the 

 Royal Society, to be recommended for election into the 

 Society. The ballot will take place on June 4, at 4 p.m. 

 We print with the name of each candidate the statement 

 of his qualifications. 



