188 Prof. A. M. Mayer on the Magnetic Elongations 



junction of the wires exposed. This apparatus was introduced 

 into the rod so that the uncovered point of the wire was about 

 one millimetre from the bottom of the hole ; and the space in- 

 cluded between the point of the wire and the bottom of the hole 

 was filled in some experiments with fine iron filings, in others 

 with mercury. The terminals of the thermo-electric couple were 

 connected with a delicate galvanometer. With the apparatus 

 thus arranged I successively made 50, 100, 200, 300, and 400 

 break- circuits, taking care that the closed circuits preceding the 

 break-circuits should all be of the same duration. After each 

 series of break-circuits the elongation produced in the rod and 

 the permanent deflection on the galvanometer-needles were noted; 

 and the observations showed that the elongations and the incre- 

 ments of temperature in the rod were proportional to the number 

 of break-circuits. 



On the elongations and retractions observed in the iron rods as the 

 strength of the magnetizing -current is gradually increased and 

 diminished ; and on the equality in the elongations produced by 

 a definite current when it is gradually and when it is suddenly 

 brought up to its maximum strength. 



The observed sudden elongations taking place in an iron rod 

 at the moment of its magnetization naturally led me to inquire 

 if the quantity of this elongation was in any way due to the sud- 

 denness of the magnetizing action, and whether the elongation 

 produced by a certain current which is gradually brought up to 

 its maximum strength would equal that produced by the same 

 current suddenly passed with the same maximum strength. 

 This problem was also connected with a proposed simple and ac- 

 curate means of measuring the changes in dimensions of bodies 

 subjected to magnetization ; and therefore I have examined it 

 with care, in the following manner. I cut the thick copper wire 

 leading from the battery to the helix, and firmly attached one of 

 its loose ends to a support. Between this copper wire and the 

 opposite wall I stretched a fine wire of German silver. The other 

 loose end of the battery wire was bent into a sharp angle, and 

 the vertex of this angle was well amalgamated. Now, by sliding 

 this bent copper wire along the fine wire of German silver towards 

 the other copper wire, I could gradually diminish the resistance ; 

 and on its touching the other end of the thick battery wire this 

 interposed resistance vanished and the current gained its maxi- 

 mum strength. On slowly retracing our steps the resistance was 

 gradually increased until the whole length of the fine wire was 

 interposed ; and then the resistance was at its maximum and the 

 strength of the current at its minimum. But if we brought the 

 two amalgamated ends of the copper wire into contact either with 



