202 ANNUAL OF SCIENTIFIC DISCOVERY. 



allowing the electric discharge to fall directly on a sensitive photo- 

 graphic plate, when it produces latent images of certain form, which, 

 under the action of the developer, yield four sharp, characteristic 

 pictures. There is a marked difference between the positive and neg- 

 ative figures, the former consisting essentially of one or more stars 

 or rings in combination, while the latter is made up, for short dis- 

 charges, of a collection of dots, or minute circles, which, by the use 

 of a greater length of discharge, became converted into two or 

 more thick concentric rings. 



NEW ELECTRO-MAGNET. 



Is it possible that our present electro-magnet is to what it might be, 

 what the cog-wheel of the early railway engineers was to the present 

 smooth one ? For after our electricians have for so many years been 

 exhausting their ingenuity to accomplish a certain object, M. Du ^Ion- 

 eel of Paris no mean authority in such matters comes forward and 

 declares that the object gained by that ingenuity is worse than useless. 

 An electro-magnet may be briefly defined to be a cylinder of iron 



/ ** *' 



covered with a helix of wire ; very powerless is the iron if no current 

 is passing through the wire ; very powerful is it witness the Royal 

 Institution magnet, and the one in Paris which is covered with 20,000 

 ft. of wire and lifts a weight of three tons while a current passes. 

 We may say, therefore, that the power of the magnet depends on the 

 wire ; and it has always been considered necessary that the wire, thin 

 or thick, according to the work to be done or the strength of the 



<5 *- 



current used, must be most carefully covered with an insulated sub- 

 stance. So we have wires covered with silk, with cotton, India-rubber, 

 and varnishes of different kinds. And this equally in the electro-mag- 

 nets used for experiments as in those used for the ten thousand purposes 

 in which electricity is now being daily employed ; indeed, we may almost 

 say that electricity works by electro-magnets. Some time ago, M. 

 Curlier, an electrician in Paris, asked himself the question Is this 

 covering necessary ? And he very properly set to work to make an 

 electro-magnet with uncovered wire to answer the question. M. Du 

 Moncel, in a communication to the Paris Academy, on the 9th inst, 

 declares that the answer thus given is so extraordinary, and the power 

 of the uncovered electro-magnet so great, that he can scarcely believe 

 his own experiments. Not only can these new magnets produce all 

 the effects of attraction of the covered ones, but the effects in some 

 cases are more than doubled. Let us produce M. Du Mono-el's figures. 

 A bar of iron 4^ centimeters long, and 7 millimeters in diameter, cov- 

 ered with a single spiral of wire 0.277 millimeters in diameter, with 

 two small Bunsei^s elements, sustained a weight of 3'U kilogrammes ; 

 covered, it could only support 2'4 kilogrammes. A larger magnet, 

 covered with 12 coats of wire, held up 9-40 grammes ; with covered 

 wire it could only support 540 grammes. The effects of distant 

 attraction Avere even more favorable. At a distance of one millime- 

 ter, and with a DaniclPs pile of 28 elements, the weights attracted 

 were as follows : Circuit. New Magnet. Old Magnet. 



kilometres 83 12 



10 " 12 3 



20 4 



