October io, 1890. J 



SCIENCE. 



199 



is then laid down, followed by the rules for predetermining 

 the iron cores and copper coils required to give any pre- 

 scribed tractive force. 



Then comes the extension of the calculation of the mag- 

 netic circuit to those cases where there is an air-gap between 

 the poles of the magnet and the armature, and where, in 

 consequence, there is leakage of the magnetic lines from pole 

 to pole. The rules for calculating the winding of the cop- 

 per coils are stated, and the limiting relation between the 

 magnetizing power of the coil and the heating effect of the 

 current in it is explained. After this comes a detailed dis- 

 cussion of the special varieties of form that must be given to 

 electro-magnets in order to adapt them to special services. 

 Those which are designed for maximum traction, for quick- 

 est action, for longest range, for greatest economy when 

 used in continuous daily service, for working in series with 

 constant current, for use in parallel at constant pressure, 

 and those for use with alternate currents, are separately con- 

 sidered. 



Lastly, some account is given of the various forms of 

 electro-magnetic mechanism which have arisen in connection 

 with the invention of the electro-magnet. The plunger and 

 coil is specially considered as constituting a species of elec- 

 tro-magnet adapted for a long range of motion. Modes of 

 mechanically securing long range for electro-magnets, and 

 of equalizing their pull over the range of motion of the ar- 

 mature, are also described. The analogies between sundry 

 electro-mechanical movements and the corresponding pieces 

 of ordinary mechanism are traced out. The course is con- 

 cluded by a consideration of the various modes of preventing 

 or minimizing the sparks which occur in the circuits in 

 which electromagnets are used. 



Historical Sketch. 



The effect which an electric current, flowing in a wire, 

 can exercise upon a neighboring compass-needle was dis- 

 covered by Oersted in 1820.' This first announcement of 

 the possession of magnetic properties by an electric current 

 was followed speedily by the researches of Ampere," Arago,^ 

 Davy,'' and by the devices of several other experimenters, 

 including De la Eive's° floating battery and coil, Schweig- 

 ger's" multiplier. Gumming V galvanometer, Faraday's' ap- 

 paratus for rotation of a permanent magnet, Marsh's** 

 vibrating pendulum, and Barlow's"' rotating star-wheel. But 

 it was not until 1835 that the electro-magnet was invented. 

 Davy had, indeed, in 1831, surrounded with temporary coils 

 of wire the steel needles upon which he was experimenting, 

 and had shown that the flow of electricity around the coil 

 could confer magnetic power upon the steel needles. But 

 from this experiment it was a grand step forward to the dis- 

 covery that a core of soft iron, surrounded by its own ap- 

 propriate coil of copper, could be made to act not only as a 

 powerful magnet, but as a magnet whose power could be 

 turned on or off at will, could be augmented to any desired 

 degree, and could be set into action and controlled from a 

 practically unlimited distance. 



^ Thomson's Annals of Philosopliy, October, 1830. 



^ Ann. de Chlm. et de Pbysique, 18:0, xv. pp. 59, 170. 



3 Ibid, p. 93. 



^ PhilosopWcal Transactions, 18*21. 



s BlbliottSque Unlverselle, March, 1821. 



« Cambridge Philosophical Transactions, 1821. 



' Quarterly Journal of Science, September, 1821. 



* Barlow's Magnetic Attractions, 18'2.3 {2d ed.). 



The electro-magnet, in the form which can first claim 

 recognition for these qualities, was devised by William 

 Sturgeon,' and is described by him in the paper which he 

 contributed to the "Proceedings of the Society of Arts" in 

 1835, accompanying a set of improved apparatus for electro- 

 magnetic experiments.' The Society of Arts rewarded Stur- 

 geon's labors by awarding him the silver medal of the so- 

 ciety and a premium of thirty guineas. Among this set of 

 apparatus are two electro-magnets, — one of horseshoe shape 

 (Figs. 1 and 3), and one a straight bar (Fig. 3). It will be 

 seen that the former figures represent an electro-magnet 

 consisting of a bent iron rod about one foot long, and half 

 an inch in diameter, varnished over, and then coiled with a 

 single left handed spiral of stout uncovered copper wire of 

 18 turns. This coil was found appropriate to the particular 

 battery which Sturgeon preferred, namely, a single cell con- 

 taining a spirally enrolled pair of zinc and copper plates of 

 large area (about 130 square inches) immersed in acid; 

 which cell, having small internal resistance, would yield a 

 large quantity of current when connected to a circuit of 



FIGS. 1 AND 2. — STURGEON'S FIRST ELECTRO-MAGNET. 



small resistance. The ends of the copper wire were brought 

 out sideways, and bent down so as to dip into two deep con- 

 necting cups, marked Zand C, fixed .-pon a wooden stand. 

 These cups, which were of wood, serred as supports to hold 

 up the electro-magnet, and, having mercury in them, served 

 also to make good electrical connection. In Fig. 2 the mag- 



1 William Sturgeon, the inventor of the electro-magnet, was born at Whlt- 

 tington. In Lancashire, in 1783. Apprenticed as a boy to the trade of a shoe- 

 maker, at the age of nineteen he ]oinf a the Westmoreland Militia, and two 

 years later enlisted into the Royal Artillery, thus gaining the chance of learning 

 something of science, and having leisure in which to pursue his absorbing 

 passion for chemical and physical experiments. He was forty-two years of 

 age when he made his great, though at the time unrecognized, invention. At 

 the date of his researches in electro-magnetism he was resident at 8 Artillery 

 Place, Woolwich, at which place he was the associate of Marsh, and was inti- 

 mate with Barlow, Christie, and Gregory, who Interested themselves in his 

 work. In 1835 he presented a paper to the Royal Society containing df scrlp- 

 tions, inter alia, of a magneto-f iectiio machine with longitudinally wound ar- 

 mature, and with a commutator consisting of half disks of metal. For some- 

 reason this paper was not admitted to the Philosophical Transactions. H& 

 afterwards printed it in full, without alteration, in bis volume of Scientific Re- 

 searches, published by subscription In 1850. Prom 1836 to 1843 he conducted 

 the Annals of Electricity. He had now removed to Manchester, where he lec- 

 tured on electricity at the Rr.yal Victoria Gallery. He died at Prestwlcb, near 

 Manchester, In 1850. There is a tablet to his memory in the church at Klrkby 

 Lonsdale, from which town the village of Whlttington is distant about two miles. 

 A portrait of Sturgeon In oils, said to be an excellent likeness. Is believed still 

 to be in existence; but all inquiries as to its whereabouts have proved una- 

 vailing. At the present moment, so far as I am aware, the scientific world la 

 absolutely without a portrait of the inventor of the electro-magnet. 



= Tranf actions of tie Society of Arts, 1825, xlili. p. 38. 



