October 26, 1888.] 



SCIENCE. 



201 



cases, however, the loss of speech brought with it the loss of musi- 

 cal expression, though it was definitely ascertained that at least two 

 of the five were musically inclined. Whether these differences 

 depend on individual education, upon different locations of the 

 affected mental centres, or upon the intensity of the affection, re- 

 mains to be determined. 



ELECTRICAL SCIENCE. 

 Electric Conductors for Alternating Currents. 



One of the most practical and useful papers read before the last 

 meeting of the British Association was one by Sir William Thom- 

 son, in which he calculated the distribution of a rapidly varying 

 electric current in a conductor. 



It is well known that an electric current which has reached a 

 steady condition in a wire is uniformly distributed through its sec- 

 tion, and the resistance of the wire varies inversely as the area. 

 But with rapidly varying currents the case is different, and the dif- 

 ference may be understood from an analogy to liquid motion, due 

 to Mr. Heaviside. In the first place. Professor Poynting has shown 

 that the electrical energy which appears in a wire carrying a cur- 

 rent is not conveyed directly through the wire from the dynamo or 

 battery supplying it, but it is first conveyed to the medium sur- 

 rounding the circuit, and then enters the wire at every point from 

 the medium. According to Mr. Heaviside, the slate of the case 

 may be partially represented by a hollow tube in a tank of water. 

 If we move the tube slowly in one direction, and if the tube be long 

 in proportion to its diameter, then in a short time all of the particles 

 of water in the tube will be moving with it, at the same velocity. 

 This represents a steady current ; and it partly illustrates Professor 

 Poynting's idea, for the motion of the water is due to the friction of 

 the tube at every point of the boundaiy, not to a pressure along 

 the tube such as would be produced by a piston in it. 



If, instead of giving a steady motion in one direction, we move 

 the tube backward and forward rapidly, we will have the outer layer 

 of water moving nearly as fast as the tube, the velocity decreasing 

 as we proceed inward ; and, finally, if we make the oscillations 

 short enough and rapid enough, the inner layers will not move at 

 all, only the particles near the outside taking part in the motion. 



Now, this is exactly what happens in the case of an electric cur- 

 rent which changes very rapidly. If the change is rapid enough, 

 the current — corresponding to the velocity of the particles of 

 water — will be mainly near the outer surface of the wire, and it 

 might happen that there is no current at all at the axis. The effect 

 of this is to increase the apparent resistance of the conductor, caus- 

 ing a greater loss from heating, and a greater fall of potential, than 

 ordinary calculation would give. 



Now, although these facts have been pretty well known since 

 Maxwell's treatise on electricity and magnetism was published, yet 

 very few people suspected that they would have any practical bear- 

 ing on alternating systems of electrical distribution. Sir William 

 Thomson, however, in calculating out some numerical examples, 

 obtains results which show that in the alternating system as ordi- 

 narily used a considerable portion of the inside of the conductors 

 does not carry any current at all, and is useless. For example : 

 with the period of alternation used by the Westinghouse Company 

 in the United States, in the neighborhood of eight thousand a min- 

 ute, the current does not penetrate so much as one-eighth of an 

 inch into the wire. The size of conductor used for distributing 

 current for even a moderate number of lamps — say, a thousand 

 lamps at a mean distance of a mile — is much beyond this limit of 

 semi-diameter, in the case cited being more than half an inch in diam- 

 eter. The result is a much greater loss by heating than is usually 

 calculated, and a fall of potential that in some cases interferes with 

 the brightness of the lamps. In order to make these effects a min- 

 imum, it would be necessary to use for conductors either thin, hol- 

 low tubes, or thin, flat strips of metal, and especially is this the 

 case when a large number of lamps are to be supplied. The ex- 

 pense of the tubes would in all probability make their use imprac- 

 ticable ; so that in future we may expect to see any extended alter- 

 nating-current distribution either with copper strips as conductors, 

 or with a number of comparatively small wires. It should be 

 pointed out as an illustration of the value of a sound mathematical 



training in applied electricity, that the best form of conductor for 

 any particular case of distribution, whether strips, a single wire, or 

 a number of wires, can be calculated from obtainable data as to 

 prices. It is the experience of the writer, however, that few electric 

 plants arc installed in a way to secure the greatest economy, and 

 much money is wasted needlessly from neglecting to make the 

 necessary calculations. 



The Sun-Lamp. — One of the most attractive of high-power 

 electric lamps is that known in France, where it was invented, as 

 the ' lampe soleiU It consists of a wedge of some refractory ma- 

 terial, marble preferably, held between two carbon rods that are 

 inclined to one another. This is set in a cavity in a marble block 

 held in an iron frame. A very simple lamp of this kind can be 

 made by boring a couple of holes in a block of marble so they are 

 slightly inclined and approach within about a quarter of an inch at 

 the bottom, and putting in two carbon rods. If this be supplied 

 with an alternating current to form an arc between the carbons, 

 the marble will be heated, and will give off a brilliant, mellow light 

 of a golden tinge, very different from the piercing but rather disa- 

 greeable light of the ordinary arc-lamp. A very high candle-power 

 can be obtained from it, and it is absolutely steady. The objec- 

 tions to its use arose from the facts that it was not certain to start 

 up automatically when the current was turned on, and it required 

 alternating currents instead of direct. This was some years ago, 

 before alternating currents had been largely introduced. From the 

 fact that a larger surface has to be heated than in the ordinary arc- 

 lamp, and the surrounding material conducts away a considerable 

 amount of heat, the lamp is not so economical as are arc-lamps. 

 In the last few years alternating-current distribution has been de- 

 veloped, and now an English syndicate is being formed to intro- 

 duce a modified sun-lamp, in which many of the objections of the 

 old form have been removed. The lighting is now automatic and 

 certain, and the lamp can be used either with continuous or alter- 

 nating current generators. For lighting halls, galleries, etc., and 

 in general for interior illumination, this modified lamp should have 

 an extensive field. 



The Electric Light vs. Gas in France. — Messrs. Brun & 

 Co., owners of a silk-manufactory at St. Clamond, give some par- 

 ticulars, in Annales Tclcgraphtques, as to the comparative cost of 

 gas and electric lights, obtained from two years' experience in their 

 works. The original lighting of the factory was by 540 gas-jets, 

 consuming 20.000 francs' worth of gas annually. These were re- 

 placed by 600 incandescent lamps. — one-half Edison, the rest 

 Swan, — the average life being 1,200 hours. The current is sup- 

 plied by an Edison dynamo of 450 amperes and 100 volts. It has 

 worked for iS months at an average of 15 hours per day. Part of 

 the factory works night and day, and some of the lamps work 

 3,600 hours a year, while others are only used for 600 hours. The 

 following are the expenses : — 



Cost of 90-horse-power engine, with fittings 33,000 francs. 



Dynamo, conductors, lamps, etc ... 23,000 " 



Total S5,ooo franc-s. 



The yearly cost is, — 



10 per cent sinking fund 5,500 francs. 



5 " " interest 2,750 " 



Increase in coal-consumption 1,300 " 



*' " oil, etc 250 *' 



Renewal, 600 lamps 2,700 ** 



Total - 12,400 francs. 



The saving is 7,600 francs per annum. The item of labor is not 

 included, as the force of mechanics was not increased on putting 

 in the installation. 



BOOK-REVIEWS. 

 The Land beyond the Forest. By E. Gerard. New York, Har- 

 per. 12". 

 Mrs. Gerard has collected her observations during a two- 

 years' life in Transylvania in the present attractive volume, and 

 greatly enhanced the value of her descriptions by adding to her 

 own experiences information from other sources, which became 



