upon the Dimeiisioiis of Iron and Steel Bars. 9.11 



which was examined by the microscope employed in tlie for- 

 mer series of experiments. Each division of the micrometer 

 passed over by the index indicated an elongation of the wire 

 under examination equal to yjoV^o^'^ of an inch. 



When pressure instead of tension was employed, a pillar 

 of iron or steel wire, one quarter of an inch in diameter, was 

 placed at C so as to support the weight of the lever; its ends 

 abutted upon flat plates of copper or brass. 



Every precaution that I could think of was taken in order 

 to give accuracy to the results. In particular I may mention 

 that the coil was not permitted to touch either the wire under 

 examination, the lever, or any other part of the apparatus to 

 which it might communicate motion through the change of 

 its own molecular condition in consequence of the passage of 

 the electrical current. In spite of these precautions the ex- 

 periments were very troublesome, owing to the almost inces- 

 sant vibrations of the index. Although my laboratory is 

 situated quite out of the town, and detached from every dwell- 

 ing, these vibrations were so extensive during the day time, 

 that the experiments had, in general, to be carried on after 

 eight o'clock p.m., when the greater part of the traffic had 

 ceased. It was at all times impossible to make an observa- 

 tion when a cart was passing along a road at the distance of 

 one or two hundred yards; nor could anything be done as 

 long as much wind was blowing. Owing to these circum- 

 stances it was frequently very difficult to estimate an effect 

 equal to one-tenth of a division of the micrometer. 1 believe, 

 however, that the results of the tables are in no case more 

 than two-tenths of a division wide of the truth. 



The experiment which I first made was with a piece of soft 

 iron wire, one foot long and one quarter of an inch in dia- 

 meter. It was made as straight as possible, and its ends were 

 ground perfectly true and flat. It was placed as a pillar under 

 the lever, so as to support its pressure, which was equal to 

 82 lbs. The coil by which it was magnetized was formed out 

 of a copper wire thirty-three yards long and one-tenth of an 

 inch in diameter, well-covered with cotton-thread. Its length 

 was \\^ inches, and its interior diameter one inch. The same 

 coil was employed in all the experiments with wires of a foot 

 long. Tiie temperature of the wire was in every case about 45°. 



The method of experimenting was the same as I employed 

 in the former section of this paper. A current was passed 

 through the coil, the effiicts of which, both on the length of the 

 iron pillar and on its magnetism, were noted. The current 

 was then cut off"; and the effect of so doing on the length of 

 the pillar noted, as well as the quantity of magnetism rcmain- 



II 2 



