56o 



NA TURE 



{Oct. 13, 1887 



natural position. Thus is represented the fact that the 

 discharge of a Leyden jar is in general oscillatory ; the 

 apparently single and momentary spark, when analyzed in 

 a very rapidly rotating mirror, turning out to really consist 

 of a series of alternating flashes rapidly succeeding one 

 another, and all over in the hundred-thousandth of a 

 second or thereabouts. These oscillatory currents were 

 predicted and calculated beforehand by Sir William 

 Thomson ; they were first observed experimentally by 

 Feddersen. The oscillations continue until the energy 

 stored up in the strained medium has all rubbed itself 

 down into heat. 



Fig. 8 shows part of an actual model of the kind. 



To make the model represent charge by induction all 

 that has to be done is to immerse a conductor into the 

 polarized dielectric — in other words, to make one or more 



Fig. 8. — Partial model of a dielectric. 



of the beads of the fixed and slippery conducting kind, the 

 other beads on the cord being of the elastic and adhesive 

 or insulating kind. Then when the displacement occurs 

 it is plain that a deficiency of cord will exist on one side 

 of the metallic layer and a surplus on the other, as shown 

 in Fig. 9. This state of things corresponds exactly to the 

 equal opposite induced charges on a conductor under 

 induction, as in Fig. 3. If the strain on one side be relieved 

 by letting the beads on that side slip back on the cord : that 

 corresponds to touching the conductor to earth, as in Fig. 4. 

 The other side has now to withstand the whole E.M.F., 

 consequently the strain there and the charge there will 

 have increased. Remove now the applied E.M.F., and 

 the negative charge appears on both sides of the metal 

 partition, either equally, or more markedly on that side 

 which has fewest beads, i.e. which is nearest to other 

 conductors. 



Fig. 9. — Metallic layer in the midst of a polarized dielectric, showing 

 opposite charges " induced" on its surfaces. (Compare Fig. 3.) 



Hydraulic Model of a Leyden Jar. — So much for the 

 cord model, but I will now describe and explain an 

 hydraulic model which illustrates the same sort of 

 facts : some of them more plainly and directly than the 

 cord model. Moreover, since all charging is essentially 

 analogous to that of a Leyden jar, let us take a Leyden 

 jar and make its hydrostatic analogue at once. 



The form of jar most convenient to think of is one sup- 

 ported horizontally on an insulating stand, with pith ball 

 electroscopes supplied to both inner and outer coatings. 



To construct its hydraulic model, procure a thin india- 

 rubber bag, such as are distended with gas at toy-shops ; 

 tie it over the mouth of a tube with a stop-cock. A, and 

 insert the tube by means of a cork into a three-necked 

 globular glass vessel or "receiver," as shown in the 

 diagram, Fig. 10. 



One of the other openings is to have another stop-cock 

 tube, B ; and the third opening is to be plugged with a 

 cork as soon as the whole vessel, both inside and outside 

 the bag, is completely full of water without air-bubbles. 



This is the insulated Leyden jar : the bag represents 

 the dielectric, and its inner and outer coatings are the 

 spaces full of water. 



Open gauge-tubes, a and b, must now be inserted in 

 tubes A and B, to correspond to the electroscopes sup- 

 plied to the jar ; and a third bent tube, C, connecting the 

 inner and outer coatings, will correspond to a discharger. 

 Ordinarily, however, of course C will be shut. 



A water-pump screwed on to A will represent an 

 electric machine connected to inner coating ; and the 

 outer coating, B, should open into a tank, to represent 

 the earth. The pump will naturally draw its supply of 

 water from the same tank. 



The bag being undistended, and the whole filled with 

 water free from air, the level of the water in the two gauge- 

 tubes will correspond with that in the tank ; and this 

 means that everything is at zero potential, i.e. the 

 potential of the earth. 



Now, c being shut, shut also B, open A, and work the 

 pump. Instantly the level in the two gauges rises 

 greatly and equally : you are trying to charge an insulated 

 jar. Turn an electric machine connected to a real jar, 

 and its two pith balls will similarly and equally rise. 



Fig. 10. — Skeleton diagram of hydraulic model of a Leyden j.-ir. 



Now open B for an instant, the pressure is relieved, and 

 both gauges at once fall, apparently both to zero. Repeat 

 the whole operation several times however, and it will be 

 found that whereas b always falls to zero, a falls short of 

 zero each time by a larger amount, and the bag is 

 gradually becoming distended. This is charge by alter- 

 nate contact. It may be repeated exactly with the real 

 jar: a spark put in to the inner coating, and an equal 

 spark withdrawn from the outer coating each time ; and 

 unless this outer spark is so withdrawn, the jar declines 

 to charge : water (and electricity) being incompressible. 



If B is left permanently open, the pump can be steadily 

 worked, so as to distend the bag and raise the gauge a to 

 its full height, b remaining at zero all the time, save for 

 oscillatory disturbances. 



Having got the jar charged, shut A, and remove the 

 pump, connecting the end of A with the tank directly. 



Now of course by the use of the discharger c the fluid 

 can be transferred from inner to outer coat, the strain 

 relieved, and the gauges equalized. But if this operatioii 

 be performed while the jar is insulated, i e. while A and B 

 are both shut, the common level of the gauges after dis- 

 charge is not zero, but a half-way level ; and the effect of 

 this is very noticeable if you touch an insulated Leyden 

 jar after it has been discharged. 



Instead of using the discharger C, however, we can 

 proceed to discharge by alternate contact, and the opera- 

 tion is very instructive. 



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