and Retractions of Rods of Iron and Steel. 193 



(2) Made circuit in outer helix ; rod elongated 1*5 division. 



fJ „ inner „ rod suddenly retracted *4 and 



then suddenly elongated *4. 

 Broke „ „ „ rod suddenly retracted *4 and 



then suddenly elongated '4. 

 u „ outer „ rod retracted 1*5 division. 



(3) Made circuit in inner helix ; rod elongated 1*4 division. 



„ „ outer „ „ „ -25 „ 



Broke „ inner „ rod suddenly retracted '35, 



then suddenly elongated *35. 

 n „ outer rod retracted 1*65. 



(4) Made circuit in outer helix ; rod elongated 1*5 division. 



„ „ inner „ rod suddenly retracted *5, 



then suddenly elongated *5. 

 Broke „ outer „ rod retracted *1 division. 



^ 



inner „ „ 1*4 



On the times occupied in the elongations and retractions of a rod 

 when the two component helices are joined as one helix and 

 placed in the circuit of one battery. 



The determinations I here give were made with the eye and 

 a chronograph, and although not as accurate as the interest of 

 the research demands, yet are near enough to the truth to show 

 that the subject is worthy of a careful investigation. The expe- 

 riments given under the above heading and the succeeding one 

 give an insight into the velocities of the molecular motions ; and 

 therefore these determinations, taken in connexion with the 

 measures of the corresponding elongations and retractions, will 

 be of considerable theoretic interest when they have been deter- 

 mined with the precision which the following proposed apparatus 

 will in all probability afford. 



I thus propose to attack this problem. The mirror of the 

 apparatus will be made of the minimum weight consistent with 

 stability. The mirror will reflect a pencil of light from an elec- 

 tric lamp to a revolving glass disk coated with sensitized collo- 

 dion. This converging pencil will form a dot of light on the 

 disk, and when the latter is stationary will, on the elongation of 

 the rod, describe a portion of one of its radii, which will appear 

 on developing the sensitized plate. If, however, the disk have 

 a uniform and known rate of rotation, the dot will, on the elon- 

 gation of the rod, describe a curved line, which, referred to the 

 appropriate ordinates, will give not only the time of the motion 

 of elongation, but also the mode or law of this motion. Of 

 course the motion of retraction can be studied in like manner. 

 Phil. Mag. S. 4. Vol. 46. No. 305. Sept. 1873. P 



