132 K H. Hall— Effect of Magnetic Force on the 



ticipated by other investigators. I desire to thank the trustees 

 of the Bache Fund for a liberal appropriation which has been 

 of great assistance in the prosecution of the work. 



About four years ago, Mr. Shelford Bid well published what 

 at first appeared to be an explanation of the so-called Hall 

 effect as being due to a thermo electric current set up between 

 strained and unstrained portions of the same piece of metal, 

 an effect which Thomson had discovered and which was well 

 known. The theory thus advanced did not stand the test of 

 examination, but it appeared from a table which Mr. Bid well 

 gave that, at least, the direction of the effect which magnetic 

 force exerts upon the equipotential lines of an electric current 

 in any given metal could be inferred from the sign of the 

 effect produced by stress upon the thermo-electric property 

 of the metal. Messrs. Coggeshall and W. A. Stone, of the 

 class of 1886 in Harvard College, working with my coopera- 

 tion, confirmed Mr. Bidwell's table in the case of copper, iron 

 and zinc, but found exceptions to it in French cold -rolled 

 steel and aluminium. No other metals were examined by 

 them. In all cases both effects were tested. A note stating 

 most of these facts was published in Science^ March 27, 1885. 



In this Journal for February, 1885, commenting upon the 

 alleged reversal of the " Hall effect," which Mr. Bidwell had 

 found in a strip of gold having two narrow longitudinal slits 

 nearly meeting, I ventured to predict the results of experi- 

 ments to be made for testing the effect of magnetic force upon 

 the equipotential lines of an electric current in strips of metal, 

 the forms of which may be called variations upon the type 

 used by Mr. Bidwell. 



For these experiments I have used untempered " French 

 cold-rolled steel." This is described by the dealer who fur- 

 nishes it as " celebrated for its toughness and superior quality 

 for striking up in die or presswork." This steel is procured in 

 the form of ribbons about 8 cm wide. From such a ribbon 

 transverse strips were cut about 21 mm wide and in length equal 

 to the width of the ribbon. They were all about -±^ m in 

 thickness. Along the middle of these strips longitudinal slits 

 were cut, in some strips one, and in others two in line separa- 

 ted by a certain space. (For illustrative figures see the article 

 mentioned above.) As all the results obtained with these strips 

 were such as might be expected from the considerations given 

 in the article alluded to, and as these considerations have not, 

 so far as I am aware, become the subject of criticism, it seems 

 unnecessary at present to describe either the apparatus or results 

 in detail. The experiments were made in February, 1885. 



Long Strip and Short Strip. — At the Philadelphia meeting 

 of the American Association in 1881 I gave reasons for think- 



