nent magnet. This was followed by Barlow's motoi" in which a si)iir- 

 shaped wheel, with current flowin,y; through it radially, rotated between 

 the poles of a permanent horse-shoe magnet. Henry, in an undated 

 letter published in SUhmatis Journ(il in July, 1831, ^ described in de- 

 tail and illustrated an electro-magnetic rocker arm machine, provided 

 with a commutator. This letter commences with the following state- 

 ment: 



"Sir: — I have lately succeeded in producing motion in a little machine by a power, 

 which, I believe, has never before been applied in mechanics — by magnetic attraction 

 and repulsion. 



"Not much importance, however, is attached to the invention, since the article, in 

 its present state, can only be considered a philosophical toy; although, in the progress 

 of discovery and iin-ention. it is not impossible that the same principle, or some modi- 

 fication of it on a more extended scale, may hereafter be applied to some useful pur- 

 pose. But without reference to its practical utility, and only viewed as a new effect 

 produced by one of the most mysterious agents of nature, you will not, perhaps, 

 think the following account of it unworthy of a place in the Journal of Science." 



The question is sometimes raised whether Henry's motor was not 

 anticipated by a similar one made by Salvatore dal Negro of Padua. 

 A review of the original publications of dal Negro, while indicating that 

 he independently invented a motor similar to Henry's, seems to indi- 

 cate clearly that his dates of publication were later. The facts are 

 these : 



The first brief description of dal Negro's motor is given in a memoir 

 entitled, "New Electro-magnetic Experiments and Observations," 

 published in Nuovi Saggi delta Imperiale Regia Accademia di Scienze 

 Lettere ed Arti in Padova, Volume HI, 1831. This volume is a collec- 

 tion of papers covering the years 1825-31, inclusive, and is one of a 

 series of volumes issued at irregular intervals of 3 to 8 years, apparently 

 for the purpose of providing a permanent record. This volume in- 

 cludes the statement that the dal Negro paper was presented before 

 the Academy of Padua on June 21 and July 10, 1831. There is no 

 indication how much later than this the volume was published, al- 

 though the character of the volume and the other evidence in this case 

 suggest that it may well have been published at the end of 1831 or later. 



In this memoir, parts 1 to 3 inclusive, comprising 16 pages and a 

 plate of illustrative figures, describe batteries and magnets. Part 4, 

 only one page in length, with no figures, describes the motor very 

 briefly as follows: 



"Part 4 — A New Electro-magnetic Motor 



Having succeeded in building temporary magnets capable of holding up consider- 

 able weights by means of electromotors [batteries] of very moderate size and therefore 

 of very small cost, I have engaged myself in the study of the way of employing this 

 new force towards setting in motion a machine of any sort, and now 1 am about to 



s S\V, Vol. 1, p. 54. 



