accuracy even of the scientists themselves is open to question where 

 matters of priority are concerned. As stated by Arago, 



"Those who discover a new fact in the sciences of observation must expect, first, 

 to have its correctness denied, — next its importance and utility contested, — and 

 afterwards will come the chapter of priority, — then, passages, obscure, insignificant, 

 and previously unnoticed, will be brought forward in crowds as affording evident 

 proofs of the discovery not being new." ' 



One hundred years later the determination of questions of priority is 

 still difficult and it is small wonder the different writers on the subject 

 of Joseph Henry's electrical discoveries have not always been in com- 

 plete agreement as to the items for which priority should be assigned to 

 Henry. In an attempt to resolve these uncertainties a study has been 

 made of the published accounts of the more important discoveries of 

 Joseph Henry in the electrical field and of the work of his contempo- 

 raries in that field. Reference has been made to the original of the 

 publications cited by earlier investigators to indicate priority or lack of 

 priority for Henry's work. 



The results of this study are given below. It is naturally not a 

 complete review of this complicated situation, relating as it does only 

 to selected items of Henry's electrical work and, furthermore, other 

 references may be found which affect the conclusions reached. It is 

 believed, however, that this study is a contribution towards a more 

 exact appraisal of Joseph Henry's work, and if it is of assistance to 

 others who may in the future wish to carry this review still further, it 

 will doubly have served a useful purpose. 



In the selection of material for this review it is the intention to in- 

 clude in it the discoveries of Joseph Henry which are the more impor- 

 tant from the standpoint of subsequent practical applications of 

 electricity. The following summary indicates briefly what these dis- 

 coveries were, and the conclusions in each case resulting from this 

 study. The somewhat voluminous reference material on which the 

 conclusions are based is given in the appendix to this paper, arranged 

 under the same numbered headings as the summary. 



Summary of Henry's Discoveries Included in This Study 

 1. Construction of Electromagnets {1829 and 1830) 



Henry used insulated wire for the winding of electromagnets instead 

 of the single spiral of non-insulated wire which had previously been 

 used, and devised windings with more than one layer. By this means 

 he was able to increase very greatly the strength of electromagnets 

 and to make them practicable for general use. His form of construc- 



1 Meteorological Essays by Francois Arago, London, 1855, p. 291. 



