280 



ELECTRICITY. 



might be indefinitely increased. "When the 

 wires forming the polar terminals of the mag- 

 neto-electric machine, were connected for a 

 short time with those of a very large electro- 

 magnet, a hright spark could be obtained from 

 the electro-helices 25 seconds after all connec- 

 tion with the magneto-electric machine had 

 been broken. Hence the author infers that an 

 electro-magnet possesses the power of accumu- 

 lating and retaining a charge of electricity in a 

 manner analogous to that in which it is re- 

 tained in insulated submarine cables, and in the 

 Leyden jar. When four magnets were placed 

 npon the cylinder, the current from the machine 

 did not attain a permanent degree of intensity 

 until after an interval of 15 seconds had elapsed ; 

 but when a more powerful machine was used 

 for exciting the electro-helices, the current at- 

 tained a permanent degree of intensity after an 

 interval of four seconds. 



Mr. Wilde then instituted experiments with 

 a large electro- magnet excited by means of a 

 small magneto-electric machine. The magnet- 

 cylinders two in number had a bore of 2 

 inches, and a length of 12J inches. Each cyl- 

 inder was fitted with an armature, round 

 which was coiled an insulated strand of copper 

 wire, 67 feet in length and 0.15 of an inch in 

 diameter. Upon one of the magnet-cylinders 

 16 permanent magnets were fixed, and to the 

 sides of the other magnet-cylinder was bolted 

 an electro-magnet formed of two rectangular 

 pieces of boiler-plate, enveloped with coils of 

 insulated copper wire. The armatures of the 

 2-J-inch magneto-electric and electro-magnetic 

 machines were driven (by steam-power) at a ve- 

 locity of 2,500 revolutions per minute. When 

 the electricity from the magneto-electric ma- 

 chine was transmitted through a piece of No. 

 20 iron wire, 0.04 of an inch in diameter, a 

 length of three inches was made hot ; when the 

 direct current from the magneto-electric ma- 

 chine was transmitted through the coils of the 

 electro-magnet of the electro-magnetic ma- 

 chine, 8 inches of the same size of wire were 

 melted and 24 inches made red hot. When the 

 electro-magnet of a 5-inch machine was excited 

 by the 2J inch magneto-electric machine, 15 

 inches of No. 15 iron wire 0.070 of an inch in 

 diameter, were melted. A 10-inch electro-mag- 

 netic machine was then constructed, the weight 

 of the electro-magnet being nearly 3 tons, and 

 of the machine 4 tons. The machine was 

 furnished with two armatures one for "in- 

 tensity " the other for " quantity " effects ; the 

 intensity armature was coiled with 376 feet of 

 No. 11 copper wire, weighing 232 Ibs., and the 

 quantity armature was enveloped with the folds 

 of an insulated copper-plate conductor, weigh- 

 ing 344 Ibs. These armatures were driven at a 

 uniform velocity of 1,500 revolutions per min- 

 ute. When the direct current from the If- 

 inch magneto-electric machine, having on its 

 cylinder six permanent magnets, was trans- 

 mitted through the coils of the 5-inch electro- 

 magnetic machine, and the direct current from 



the latter transmitted to the electro-magnet of 

 the 10-inch machine, an amount of dynamic 

 electricity was evolved from the quantity ar- 

 mature sufficient to melt pieces of an iron rod 

 15 inches long, and fully one-fourth of an inch 

 in diameter. With the same arrangement the 

 electricity from the quantity armature melted 

 15 inches of No. 11 copper wire, 0.125 inches 

 in diameter. When the intensity armature was 

 placed in the magnet-cylinder, 7 feet of No. 16 

 iron wire were melted, and 21 feet of the same 

 wire made red hot. The illuminating power 

 of the electricity from the intensity armature 

 was so great as to cast shadows from the flames 

 of the street lamps a quarter of a mile distant. 

 When viewed from that distance, the rays pro- 

 ceeding from the reflector were said to have all 

 the rich effulgence of sunshine. A piece of 

 ordinary sensitized paper, when exposed to the 

 action of the light for 20 seconds, at a distance 

 of two feet from the reflector, was darkened to 

 the same degree as when a piece of the same 

 paper was exposed for one minute to the direct 

 rays of the sun on a clear day in the month of 

 March. 



These extraordinary powers of the 10-inch 

 machine are all the more remarkable, be- 

 cause they have their origin in six small per- 

 manent magnets, weighing only 1 Ib. each, and 

 capable collectively of sustaining only 60 Ibs., 

 while the electricity from the magneto-electric 

 machine is of itself incapable of heating to red- 

 ness the shortest length of iron wire of the 

 smallest size manufactured. 



Thermo- Electricity. M. Becquerel has re- 

 cently published an elaborate memoir on the 

 thermo-electric power -of different alloys and 

 the construction of thermo-electric batteries. 

 For thermo-piles of low temperatures he rec- 

 ommends an alloy consisting of equal equiva- 

 lents of antimony and cadmium, with one-tenth 

 the weight of bismuth for the positive metal, 

 and an alloy of ten of bismuth to one of anti- 

 mony for the negative metal. For piles of a 

 high tension, the negative should be of German 

 silver, and the positive may be either the above 

 mixture of antimony, cadmium, and bismuth, or 

 fused and annealled sulphide of copper ; the lat- 

 ter stands the greatest heat, but also gives the 

 highest resistance. Sulphide of copper being a 

 very bad conductor of heat, it will scarcely be 

 found necessary to cool the. other ends ; but this 

 should be done when a 'metal is used, and the 

 length of the bar should in that case be in- 

 creased. Thermo-electric piles, on account of 

 their low tension, cannot yet replace hydro-, 

 electric batteries, but for special purposes, and 

 particularly for the study of radiant heat, they 

 offer new facilities. 



Electric Conductivity of Gases under feebl& 

 Pressures. M. Morren communicates to the 

 Annales de Chimie et de Physique, some ac- 

 count of his experiments on the electric con- 

 ductivity of certain gases. For each gas there 

 is a certain pressure, at which the conductivity 

 is at its maximum, as follows: 



