184 



KNOWLEDGE 



[August 1, 1900. 



state that lichens gi'owiug on a surface of pure cjuartz 

 may derive the very small amount of mineral nutriment 

 required by them from blown dust, which easily accumu- 

 lates about them, though in far too slight a quantity 

 to form a soil. It may be remembered also that, while 

 quai-tz-veins are fairly pure, quartzites contain a number 

 of other scattered minerals besides quartz ; hence some 

 massive rocks, practically composed of quartz, may yet 

 sei-ve as a ground for the growth of lichens. 



Grenville a. J. Cole.] 



WIRELESS TELEGRAPHY.-III. 



By G. W. DE TUNZELMANN, B.SC. 



MECHANICAL REPRESENTATIONS OF 

 ELECTRIC ACTIONS. 



In my last article* some account was given of the 

 medium through which electric actions are transmittecl. 

 All that I can do towards exjDlaining the mechanism 

 of transmission is to lay before my readers mechanical 

 arrangements cajiable of producing the observed results, 

 and which may not impossibly bo something like the 

 real ones, which a,s yet remain hidden from our view. 



Clerk Maxwell was the first to give helpful suggestions 

 in the way of mechanical models illustrating electric 

 actions. His model was modified and imjoroved by 

 Professor Fitzgerald, and later Professor Oliver J. Lodge 

 treated this question in a most exhaustive manner in a 

 series of papers extending over the last twenty years. 

 It is upon the store the latter has provided that I shall 

 mainly draw at present. 



We may consider ourselves as living in a sort of ocean 

 of electricity, and as water is so much more familiar 

 to us than electricity, it may help our conceptions to 

 imagine for a moment that we are living under the sea, 

 and consider the water as taking the place of the ocean 

 of electricity really surrounding us, but the analogy then 

 will not be quite complete. Water can be displaced 

 by solid and other bodies, whereas in the electric ocean 

 the amount of electricity contained within a given space 

 is just the same, whether part of that space is occujjied 

 by matter of any kind, or whether it is what we call 

 empty. 



There is no possibility of putting electricity into any 

 body in the way that we fill a bucket with water. 

 When living under the water our buckets must always 

 be full of water. We may change the water, but only 

 by displacing it by an equal quantity from some other 

 portion of the all-pervacling ocean. As water cannot 

 pass through the sides and bottom of the bucket, so in 

 the same way insulators will not allow electricity to 

 pass through them. The space occupied by the material 

 of the bucket is not full of water, while the space 

 occupied by an insulating material contains exactly 

 the same amount of electricity as if the material were 

 not there; but this will not seriously affect the use of the 

 analogy. There is another and far more important 

 difference between the two oceans ; of water, and of 

 electricity. While the water can move freely from one 

 part of the ocean to another it is not so with electricity 

 in so called empty space, this being an insulator. We 

 find that electric waves can be sent through space, and 

 therefore some small backward and forward motion must 

 be possible but no continuous flow. The electric ocean 

 must therefore be considered as entangled in a sort 

 of jelly, which will allow of slight vibratory displace- 



* Knowlkuqe, May, 1900. 



ments, but in order to get a continuous flow some means 

 of making tubes or channels in the jelly must be found, 

 and these are called conductors. 



In order to get a flow through a tube we must have 

 some means of driving the electricity along, such as 

 would be provided by a pump in the case of water, and 

 for the flow to be continuous the tube must form a 

 closed circuit. An analogy for a circuit composed 

 partly of conductors and partly of insulators may be 

 found in an endless tube containing diaphragms of some 

 elastic substance, such as indiarubber, stretched across 

 it at intervals. 



A section of such a tube is shown in Fig. 1. The 

 pump. A, has valves so adjusted as to send a current of 

 water down the tube in the direction shown by the 

 arrows. At B is an elastic diaphragm stretching across 

 the tube. Before the pump is worked the diaphragm 

 will be subject to equal pressure on opposite sides, and 

 will therefore remain flat, as shown in section by the 

 straight line, a, b, c. By working the pump the pressure 

 on the right-hand side of the diaphragm will be increased 

 and that on the left diminished, and the diaphragm will 

 be bent into some such form as is represented by the 

 curved line. If diaphragms are placed across the tube 

 at each side of the pump. A, and the jDump is then 

 removed, and the tube divided through the diaphragm 

 so as to leave each end enclosed, two tubes with their 

 diaphragms will appear, as shown in Fig. 2, and the 

 right-hand half of the tube will contain more and the 

 left-hand half less than half of the water originally in 

 the tube. These two tubes represent two equally and 

 oppositely charged conductors, and it will be seen that 

 according to this representation it is impossible to charge 

 a conductor positively without at the same time giving 

 an equal negative, or opposite charge, to some other 

 conductor. 



The greatest visible effects are produced when the 

 whole of one conductor is brought as near as possible 

 to the whole of the other while maintaining insulation 

 between them. This is veiy conveniently done by 

 pasting sheets of tinfoil on the inside and outside of a 

 glass jar or on the opposite faces of a sheet of glass, 

 forming the well-known Leyden Jar, and it will be seen 

 that this consists simply of a pair of conductors insulated 

 from each other, and that the case of charging or dis- 

 charging any conductor is a case of the charge or 

 discharge of a Leyden Jar. 



Professor Lodge has designed an elaborate model of 

 a Leyden Jar ; Fig. 3 being a skeleton diagram, and 

 Fig. 4 an illustration of the actual model. 



A thin indiarubber bag is tied over the mouth of a 

 tube provided with a stopcock, A, and the tube is inserted 

 by means of a cork into a three-necked globular glass 

 vessel. One of the other openings must have a stop- 

 cock, B, while the third opening is closed with a cork, 

 or preferably another stopcock, as soon as the whole 

 vessel, both inside and outside the bag, is filled with 

 water fi'ee from bubbles of air. A third tube, usually 

 closed by a stopcock, C, represents a discharger; and 

 open gauge tubes, a and b, represent electroscopes 

 attached to the two coatings of the jar respectively, 

 while a water pump screwed on to A corresponds to 

 a source of electricity, such as a battery, or a frictional 

 or influence machine. If the two terminals of the 

 source ai-o attached to the two coatings of the jar, then 

 A must be connected to B by means of a tube, while if 

 one terminal of the source and one coating of the jar are 

 connected with the earth, the more usual arrangement, 

 then A and B must both be connected with a tank of 



