1S21.] Historical Sketch of Electro-magnetism. 277 



merits, to expect a much greater effect if the connecting wire were 

 put into the form of a spiral, and the piece to be magnetised 

 were placed in its axis. According to the theory, in a needle or 

 magnet, pointing to the north, the currents were in the upper 

 part from east to west. In consequence of these expectations, 

 MM. Ampere and Arago made experiments with spirals or 

 helices, and the results are mentioned in M. Arago's paper, on 

 the Communication of Magnetism to Iron Filings, published in 

 the Annales de Chimie, xv. 93 ; so that probably the experi- 

 ments date from Sept. 25, though there is no date to the paper. 



On twisting a wire round a rod, it may be made to pass either 

 in one direction or the other, giving rise to two distinct but 

 symmetrical helices, which have been named by botanists dex- 

 tror&um and sinistrorsum. Though their diameters be equal, and 

 the spirals which compose them have equal inclinations, yet they 

 can never be superposed ; for however they are turned about, 

 their direction is the same. The dextrorsum, or, as we may call 

 it, the right helix, proceeds from the right hand downwards 

 towards the left above the axis ; the tendrils of many plants 

 exhibit instances of it, and it is almost exclusively used in the 

 arts : the sinistrorsum, or left helix, proceeds from the left hand 

 downwards towards the right above the axis. 



Having made some of these helices, one was connected by 

 its extremities with the poles of a voltaic battery, and then a 

 needle wrapped in paper placed within it ; after remaining there 

 a few minutes, it was taken out, and found to be strongly 

 magnetized ; and the effect of a helix above that of a straight 

 connecting wire was found to be very great. 



Then with regard to the position of the poles in the magnetised 

 needle, it was found that whenever a right helix wasjjs€*i, that 

 end of the needle towards the negative end of the battery pointed 

 to the north, and that towards the positive end toward the south; 

 but that with a left helix, that end of the needle towards the 

 positive, pointed north ; and the other end, south. 



In order to establish this point, the connecting wire was 

 sometimes formed into one helix, sometimes into two or three, 

 which was readily done by twisting it round a glass tube, or rod, 

 first in one direction, then in another ; and when needles pre- 

 viously inclosed in glass tubes were then placed in these helices, 

 the magnetic poles they received were always in accordance 

 with the statement just given. In one case, also, where the 

 connecting wire had been formed into three consecutive helices, 

 the middle one being of course different to the other two, a single 

 piece of steel wire sufficiently long to pass through all three of 

 them being inclosed in a glass tube was placed within them. 

 On being again removed, and examined, it was found to have 

 six poles: first, a north pole, a little further on a south pole, then 

 another south pole, a north pole, another north pole, and at the 

 further end a south pole. 



