and Retractions of Rods of Iron and Steel. 195 



induced currents. For the present I am content with merely 

 presenting tJie facts ; for I have not yet been able to command 

 the time which their investigation will require. 



In experiments (3) and (6) the times of retraction were respec- 

 tively lyV °f a second and 1 T 3 F of a second; and the slow- 

 ness of these motions allowed me to obtain an insight into their 

 character. In each of these experiments the rod retracted with 

 a gradually diminishing velocity, and the motion reminded one 

 forcibly of that pertaining to a body projected vertically upwards. 



The coefficients of elongation and of retraction of seven rods of 

 different species of iron, and of three steel rods of various de- 

 grees of hardness. 



It remains to give the determinations I have made of the 

 coefficients of elongation and of retraction. These measures 

 were made on rods of circular section, 60*1 inches long and *5 

 inch in diameter. As previously stated, the iron rods were 

 thoroughly annealed, and the steel rods were carefully tempered. 

 On the ends of the rods numbers were stamped ; and these marks 

 corresponded to the rods as follows : — 



1 . 



, . Scrap iron. 



2 . 



. Ulster iron. 



3 . 



. Norway iron. 



4 . 



. English refined iron. 



5 . 



. Low-Moor iron. 



6 . 



. . Fall-River iron. 



000 . 



. . Steel, soft. 



00 . 



. „ „ hardened and drawn to blue. 



. 



. . „ „ „ yellow. 



The method of determining these coefficients was as follows : — 

 When the rod had attained a fixed temperature, so that the 

 scale-reading remained constant for an hour, I recorded this 

 scale-reading. I then passed the current from the 25-cell bat- 

 tery ; and as soon as the new scale-reading thus produced was 

 read, I broke the circuit and obtained the corresponding scale- 

 reading. These readings were now written in the note-book ; 

 and immediately after recording them I again made and broke 

 the circuit, and noted the two corresponding readings of the tele- 

 scope-scale. I then continued making and breaking the circuit 

 and recording the scale-divisions until the rod began to elongate 

 from the heat produced on demagnetization. 



The Tables following (see p. 200) consist of six columns, A, B, 

 C, D, E, and F. Under A are designated the rods. B contains 

 the elongations or retractions produced on first passing the cur- 

 rent ; C the retractions or elongations observed after the first- 



P2 



