198 



♦ KNOWLEDGE ♦ 



[Skpt. 4, 1885. 



openings vertical. If now we imagine our model of a 

 beam of ordinary light, V^, Fig. 45, to be pushed ("end 

 on," as sailors say) against such a grating it is abun- 

 dantly evident that, while the vertical series of vibra- 

 tions, A B, would pass through perfectly, the trans- 

 verse ones, C D, would be most effectually stopped : 

 just as a walking-stick, held parallel to some iron 

 i-ailings, could be passed easily between them, but would 

 be arrested at once if held across the direction of theii- 

 length. At A, in Fig. 48, the, so to speak, cross 

 vibrations C D, Figs. 44 and 45, have been shut out by 

 the first plate, abed, and only the vertical ones, AB 

 (same figs.) allowed to pass ; and as our hypothe- 

 tical "grating" is also vertical in the second plate, 

 efgh, the vertical undulations will get through that 

 perfectly. But (B, Fig. 48) as the vibrations which pass 

 through abed are all vertical, and vertical only, when 

 we make our second supposititious grating, efgh, hori- 

 zontal, of course we interpose an insuperable barrier to 

 their passage, and darkness is the result. 

 (To he continued.) 



THE YOUNG ELECTRICIAN. 

 By "W. Slingo. 



(Continued from p. 158.) 

 ELECTROSCOPES. 



PR. 6. — Electroscopes are instruments for detectincr 

 the presence of a charge of static electricity, and 

 for determining its quality. They are of a simple nature, 

 and exhibit important and interesting phenomena, but 

 they must not be confounded with electrometers or 

 instruments for measuring the strength or quantity of a 

 charge. Induction is the ruling principle in the construe 

 tion and action of electroscopes. 



Ex. ClY. — Anything which will indicate the jiresence 

 of electricity (such as a suspended feather, pith-ball, ic ) 

 is in a measure an electroscope ; but we will confine our- 

 selves to such instraments as respond to the more com- 

 plete definition. Fig. 58 illustrates a typical form of 

 instrument. It consists of a bottomless gas-receiver or 

 glass-jar, F, in the neck of which is a hard- wood stopper 

 or cover, D, cemented on with sealing-wax, &c., and 

 through a hole in the centre of which is a glass or other 

 insulating tube, C. Through C passes a brass rod, B, 

 secured to a brass plate (or ball). A, at the top, the lower 

 extremity being flattened out or attached to a flat cross- 

 jiiece of metal, E. On to the sides of E gold-leaves or 

 pieces of Dutch metal, L L, are gummed, or otherwise 

 attached. 



Ex. CV. — A somewhat more elaborate instrument is 

 shown in Fig. 59, ia which A is the plate, B the insu- 

 lated rod, and LL the leaves. In addition a metal 

 shield, G, is fitted to the upper portion of the glass jar. 

 It is insulated from the rod and plate A, but is connected 

 to the earth or some other conductor in comiection with 

 it. Two strips of tinfoil, K K, are gummed or pasted on 

 the inner surface of the jar, reaching at least high 

 enough for the leaves, L L, to touch them instead of the 

 glass (in the event of their being so far separated). These 

 strips are also connected to earth. A small glass tray, 

 H, of any convenient shape or dimensions is laid on the 

 wooden base, W, and is supplied with a few small pieces 

 of pumice-stone saturated with sulphuric acid, the func- 

 tion of which is to absorb any atmospheric moisture that 

 may be present. 



Kx. C\'i.- A f'l-m frequently recommended for 

 amnt,-m-s ,,■ l.-_•>lln..■^ i\ that illustrated in Fig. GO, 

 where !■' is a l-Ii-s tli-k, which has been thoroughly 

 dried auj waniii'l. .mm] in the neck of which is a .sound 

 cork or %iile:iiiii- t. ]mm r, (', through which is passed a 

 wire, W, the iii i . i j ;r' . f which is soldered to a small 

 disc, T, the low, ]■ juit, \V\ heing bent at right angles to 

 carry the leaves, L L. 



Ex. CVII. — For our present purpose I have not a 

 great opinion of either of these pieces of apparatus. 

 They are one and all more costly or more elaborate than 

 is necessary. The object of the glass jar or flask is 

 simply to protect the leaves from air-currents, and prevent 

 their being affected by atmospheric moisture. If we 

 could get a transparent metallic jar, nothing could suit 

 our purpose better, but that, of course, is out of the 

 question. Let us study Fig. 58 and its teachings a little, 

 and we shall then see what is actually required. Suppose 

 an electrified rod, say of glass, to be brought near the 

 plate A. Induction takes place, A becoming negatively, 

 and the leaves L L positively, charged. L L, in conse- 

 quence of their similar charges, mutually repel, and, 

 instead of hanging vertically, take up positions more or 

 less resembling those illustrated, the amoant of divergence 



Fig 5 



Fig 50 



Fig bO 



in any particular instrument depending naturally upon 

 the charge. On the electrified rod being withdrawn, the 

 leaves fall together, exhibiting no signs of electrification, 

 the equal charges produced at A and L L combining and 

 producing neutrality again. Let the rod be once more 

 brought towards A, and the electroscope re-charged ; then 

 (the position of the rod being unaltered) let the finger 

 touch B, or any part of A not immediately opposite the 

 rod. The leaves will fall together consequent on the 

 positive electricity repelled to them being neutralised by 

 the earth-contact through the finger. As a matter of 

 fact, the state of affairs is somewhat akin to a huge con- 

 ductor, of which one extremity is at A and the other at 

 some antipodean point. 



Remove the finger. No change takes place in the 

 position of the leaves. Subsequently remove the glass 

 rod and the leaves diverge : this time with negative 

 electricity, because the removal of the rod "sets free" the 

 negative charge concentrated at A, some of which charge 

 finding its way to the leaves causes the divergence. A 

 finger placed on A neutralises this negative charge. 

 Suppose the finger not to be so placed, then we have a 

 charge of electricity in the leaves whose quality (positive 

 or negative) we know, because we first u.sed a body 

 provided with a known charge. 



To determine the quality of another charge all that is 

 necessary is to place the body carrying the charge near 

 A, and inducing a further charge in the metallic system 

 A L. If the unknown charge is negative, a positive 



