March 10, 1882.] 



♦ KNOVV^LEDGE ♦ 



409 



The magnets may be from 4 to 6 in. long, and j to i in. in 

 diameter, and ^rill cost per pair from 2s. to 2b. 6d. The coils of 

 fine wire, the same as magnets, and the iron, which is known as 

 ferrotype, about 4d. per pair. The cases, if bought ready turned, 

 will cost about 6s. the pair. Now, as this is the n\ost expensive item, 

 I will show that ^vith a little ingenuity this may be overcome at about 

 one-tenth the above price. At any chemist's procure a couple 

 of empty violet-powder boxes, at the outside fourpence, and see 

 that the lids fit tightly. Then, in the centre of the lid, bore a hole 

 i of an inch in diameter, then pare away carefully all around the 

 kole until you get a shallow, funnel-shaped mouthpiece. Finish off 

 with a piece of glass-paper. Now cut a ring of cardboard, J of an 

 inch wide, that will just lit inside the lid of your box. Take one of 

 the pieces of iron, and, with a pair of scissors, cut a disc that will 

 also fit inside the lid of box, ilrop in the ring of cardboard, and 

 next the iron disc, and see that the cardboard is thick enough 

 to prevent the disc from touching the lid anywhere but at the 

 edges, the middle being free to vibrate. In the bottom of the 

 box, bore a hole just large enough to take the magnet easily. You 

 now want some sort of a support for the rest of the magnet, and 

 also to form a handle. This can bo formed from a round piece of 

 wood about 3 in. long and 1 in. in diameter, with a hole through the 



centre lengthwise to take the magnet. This must be glued to the 

 bottom of the box, the holes corresponding ; or, should you find any 

 difficulty in shaping the stem, take a large cotton reel, the end of 

 which will form a sort of flange, and will bo better for glueing. 

 You are now ready for putting the various parts together. Push 

 the magnet up through the reel until the end projects 

 within the box, and stands almost level with the edges of 

 the box without the lid ; slip the bobbin of wire over end of 

 magnet in box, and fix it so that about -f^ of an inch of the 

 magnet is above the reel or bobbin. Pass the two ends of the fine 

 wire through bottom of box, and solder to two brass binding screws 

 fixed somewhere on the case, tlie proper place being at the end of 

 them. Put the lid (with diaphragm and ring in place) on the box. 

 The diaphragm will now be firmly pinched between the lid and box, 

 and should be just clear of the end ot magnet. Your telephone is 

 now complete, and can be polished, or varnished with a spirit 

 varnish, and when dry is ready for nse. To connect, simply 

 take a wire from each binding-screw on one telephone, and 

 connect to each binding-screw on the other, and the instruments 

 will speak. But beware of a very common error, and don't try 

 to speak into one and hear your o^vn voice in the other. This is 

 impossible. Have at least a dozen yards in each screw, and leave 

 one instnunent with a frienji while you take the other into another 



room. Speak distinctly into the mouthpiece, and when hearing keep 

 the mouthpiece tight up to your car. For connecting, use cotton- 

 covered copper about No. 20. 



Should the voice seem very distant, move the magnet nearer the 

 diajiliragm. If no voice is heard, you vrill most likely find the 

 magnet sticking to the diaphragm, the proper distance being about 

 the thickness of a piece of stout paper. 



As now used and sent out by the different telephone companies, 

 the telephone is never used both as a receiver and transmitter. 

 The transmitting instrument being a microphone of peculiar con- 

 struction, this adds greatly to the utility of the instrument, as you 

 are able to carry on conversation without removing the receiving 

 telephone from your ear, all the speaking being dono at the 

 microphone. 



I shall be most happy to send a description of the microphone, 

 both simple and as a transmitter, as made by G. E. T. 



PROBABILITIES IN CARD DRAWING. 



[324] — Chas. A. Edes writes, with reference to the problem in 

 probabilities at p. 301, that the cards are not shuffled between the 

 cuts, so that the cutter, if he fails the fii-st time, has a rather better 

 chance next time, because ho will not cut in the same place again, 

 and similarly for the third trial. To solve the problem in this 

 form, treat it as though the card cut were removed from the pack 

 at each failure. The solution would then run thus : — 



The chance that a winning card will not bo cut the first time ia 



— . In that case there remain 51 cstrds, of which 12 are winning 

 ones. Thus, the chance that there will be a second trial, and that a 



winning card will then not be drawn, is — x — There then remain 



13 51 

 50 cards, of which 12 are winning ones ; and the chance that there 

 will be a third trial, and yet again a winning card not be drawn, ia — 

 lu 39 ,38 



is'^'sT^so" 



^38 

 85 

 The odds in favour of drawing one of the winning cards at one of 

 the three trials are therefore 47 to 38, instead of the smaller odds 

 1,197 to 1,000. It is obvious, of course, that in the closing sen- 

 tences of the examination of the problem at page 301, I should have 

 written " in favour of A " instead of " against A." Editor. 



CENTRIFUGAL FORCE AND AN OPTICAL ILLUSION. 



[325]. — When a piece of twine is attached at one end to a ring, 

 and the other end, held between finger and thumb, twisted and 

 untwisted rapidly, the ring, when it has acquired sufficient velocity, 

 spins horizontally in either direction, according to the twist given 

 to the tAvine, and the eye is deceived by the appearance of a second 

 string attajhed to the opposite side of the ring, the ring and two 

 strings forming a perfect cone, whose apex touches the finger and 

 thumb. A gutta-percha ring, about 2 in. in diameter, shows this 

 simple exiieriment to advantage. E. C. 



[This is a very pretty illusion. If the weight of ring is small, 

 we see a nodal chord, generally a third of the wny up. The 

 apparent cone is not perfect. It is easily seen why the cone seems 

 bounded by two distinct strings. — Ed.] 



Sign of " Beas and Ragged Stafp." — Shakespeare refers to 

 this as the crest of Warwick, " the king-maker," in 2 Henry VI., 



act v., sc. 1, where Warwick is made to say : 



" Now, by my father's badge, old Nevil's crest, 

 The rampant bear chain'd to the ragged staff." 



— W. MiDDLETOX BOTTER.S. 



I.NTELLiGEN-CE !:< ANIMALS. — About twelve years since, when I 

 was living at Richmond, I had a fine black retriever. She was an 

 exceedingly docile, tractable animal with people, but always 

 shunned animals, even those of her o^vn species. One day, walking 

 with her through the streets of the town, she was i)ersecnted by a 

 large, coarse, ill-bred, bull-dog ; she snapped at him several times. 

 Presently I turned down to the water-side ; to my surprise, here she 

 began playing with the dog, and soon after she gambolled about in 

 shallow water with him, occasionally swimming. All at once ha 

 got out of his depth, and, to my astonishment and dismay, she got 

 him by the nape of the neck and kept his head under water. I 

 shouted *' Down, Prin, down ! " but ot" this she took no notice, until 

 she had almost suffocated him ; then she let him go, and ho dragged 

 himself out of the water, and slunk away with his tail between hia 

 legs, and went his way, a sadder, and I hope a wiser, dog. — J. 

 Bbownino. 



