296 



SCIENCE. 



[Vol. XI. No. 281 



advance had moved eastward, to be felt over the British Isles from 

 the 17th to the 21st of the month, and after it a rapid fall of the 

 barometer. The isotherm of 32° reached from the southern coast 

 of North Carolina well offshore, thence northward to the coast of 

 Maine, and from central Maine eastward across Cape Breton 

 Island and southern Newfoundland. From the south-eastern to 

 the north-western portion of the chart, the shades of color showed 

 a difference of temperature of more than 80° (from above 70° to 

 below— 10°); but such great differences of temperature and press- 

 ure could not last long, and the normal conditions were gradually 

 restored. 



ELECTRICAL SCIENCE. 



Atmospheric Electricity. 



The London Electrician contains an abstract of a paper by Prof. 

 L. Weber which is of interest. He erected two insulated conduct- 

 ors on the top of the Riesengebirge ; but he says, that, curiously 

 enough, since they have been put up, they have never been struck 

 by lightning, although before their erection lightning-flashes were 

 continually occurring. He also made some kite and balloon e.xperi- 

 ments, in connection with which he goes at considerable length 

 into the question of the effect of the conducting-string in altering 

 the electrical condition of the circumjacent air layers, and also con- 

 siders the effects due to a long conductor completely insulated from 

 the earth, and without discharging-points ; a similar conductor, 

 with slight power of discharge along its whole length ; an insulated 

 conductor, with strong discharge-power (e.g., a flame) at the up- 

 per end ; and other similar and more complicated cases. His kite- 

 string was really a steel wire. The discharge-points of the kite 

 consisted of 400 needle-points. In other cases he had the tails of the 

 kite made of silver paper for the same purpose. The potential was 

 measured by the length of sparks ; the current, with a galvanom- 

 eter. The latter varied in general from .07 to 2.5 micro-amperes. 

 The potential varied generally from 3,000 to 10,000 volts. When 

 thick clouds were overhead, there were no appreciable sparks, the 

 strongest sparks being obtained when the zenith was either quite 

 clear, or when cumulo-stratus clouds appeared. With potentials of 

 11,000 and 20,000 volts, currents of 4 and 8 micro-amperes were 

 obtained. 



Incandescent Lamps with Alternating and Direct 

 Currents. — Professors Ayrton and Perry have carried on a 

 series of experiments to determine whether the efficiency of incan- 

 descent electric lamps is the same when supplied with alternating 

 currents and with direct currents. The following table gives the 

 results of measurements on four different lamps : — 



Lamp. 



No. of Experi- 

 ments made. 



Watts per Candle. 

 White Light. 





Continuous. 



Alternating. 



X 



.0 



3 -053 



3-033 





19 

 16 



Gree7i Light. 



Red Light. 





Continuous. 



Alt. 



Continuous. 



Alt. 



3 



4 



2-597 

 2-935 

 2,900 



2-534 

 2 966 

 3-073 



3-100 



3-254 

 3-504 



3.100 

 3-164 

 3-477 



Mean of last three experiments... , 



2. 811 



2-857 



3.286 1 3.247 





Continuous. 



Alternating. 



Mean of all results 



3.0 



t9 



3.04 



97 



These results show, that, as far as the economy of the lamp is con- 

 cerned, the efficiency of the two systems is about the same. What 



the life of the lamp would be with alternating currents is a matter 

 which has yet to be decided. Considering the rapidity with which 

 small wires respond in temperature to changes in current, it might 

 be, when the period of the alternating current is not extremely 

 rapid, that the filament of a lamp supplied by such a current would 

 be at times at a much higher temperature than the average, at 

 other times at a lower temperature. If this were the case, we 

 would expect that the life of a lamp supplied in this way would be 

 less than that of the same lamp fed by a continuous current. With 

 300 reversals a second, however, the temperature would vary but 

 little, and there is no reason that the life of the lamp should not be 

 the same with continuous and alternating currents. 



Polarization of Platinum Plates. — Mr. C. H. Draper 

 has experimented on the electro-motive force of polarization be- 

 tween platinum plates immersed in dilute sulphuric acid, for differ- 

 ent strengths of current passing between the plates, and with dif- 

 ferent temperatures. It is well known, that, if an electric current 

 be sent between such plates, an electro-motive force of polarization 

 is produced, in a direction opposite to that of the impressed electro- 

 motive force, and of a value something in the neighborhood of 

 one and a half volts. Mr. Draper tried to find if this opposing 

 electro-motive force was independent of the current and tempera- 

 ture, and, if not, in what way it varies with them. The conclusions 

 at which he arrives are as follows : I. The opposing electro-motive 

 force of polarization which arises in cells when at work depends on 

 the value of the current passing through them when that current is 

 below a certain value, increasing, but more and more slowly, with the 

 current ; 2. There is a maximum value of the polarization regarded 

 only as a function of the current strength, beyond which any in- 

 crease in the strength of the current has no effect upon it ; 3. The 

 electro-motive force of polarization varies with temperature, its 

 value decreasing about one per cent for a rise of temperature of 

 406. 



Electric Mining Road AT Lykens. — Among the interest- 

 ing applications of electricity to mining-work, the electric road in 

 the coal-mines at Lykens, Penn., is one of the most successful. It 

 has been pointed out in this journal that electricity offers especial 

 advantages for use at mines where fuel is scarce and water-power 

 of easy access, as in the silver and other mines in our Western 

 territory; but, besides the decreased cost of fuel, the ease with 

 which electric motors can be used in almost any position, under 

 conditions that steam-engines could not meet, makes electric trans- 

 mission still more valuable. In coal-mines the cost of fuel is, of 

 course, a small item ; but the greater safety, efificiency, and flexi- 

 bility of a system of electrical distribution, as compared with a 

 number of steam-engines, give it an advantage which must soon be 

 recognized. In the Lykens Valley Mines there has been used for 

 some time an electric-motor car to take the place of mules for 

 hauling cars from the mine. The length of the road is 6,300 feet ; 

 the weight of the locomotive, 15,000 pounds ; the largest load it is 

 capable of handling, 150 tons ; the speed, 6 to 8 miles per hour. A 

 second road on the same general plan is being equipped for the 

 same company. The system employed is the Schlesinger. 



Designing Dynamo-Electric Machines. — Until very re- 

 cently the designing of dynamo-electric machinery was an em- 

 pirical matter. The practice was to roughly guess, from the 

 dimensions of some similar machine, about what the dimensions 

 should be to give the required output, and, after the dynamo was 

 built, to change the number of revolutions or the winding of the 

 field-magnets until the required conditions were fulfilled. Some- 

 times even this would not suffice to bring the machine to its output, 

 in which case another was built. In the last two years the papers 

 of Mr. Kapp and Dr. Hopkinson, together with the growing habit 

 of treating a magnetic circuit in the same way that ordinary electric 

 circuits are treated, introducing the idea of magnetic resistance, 

 have greatly increased the certainty with which dynamos may be 

 designed. In fact, from experiments on one machine of a type, we 

 can design another of the same type to give any required output, 

 with considerable accuracy. While this is not generally recognized 

 in this country, it soon will be, and a great deal of expense and 

 energy will be saved ; besides which, a consideration of the mag- 

 netic resistance of various parts of the magnetic circuit of a dynamo 



