Molecular Change in Iron Wire. 61 



secured to it, which dips into a little shallow dish or cup of mercury, 

 J ; and the mercury in this cup is connected by a screw and strip 

 of brass to the binding-screw C. K is a stretched band of vulcanized 

 india-rubber, attached at one end to the hook of the wire E, and 

 at the other end to the hook L (see fig. 2). The cylindrical binding- 

 screw I has a hook by which it is attached to the loop M (fig. 2). 

 N is an axis suspended delicately upon centres, and carrying a very 

 light index pointer O. The hook L and loop M are separate pieces of 

 metal, and move freely upon an axis, P (fig. 2) . The distance from 

 the centre of the axis N to that of P is 12*72 millimetres (=0*5 

 inch), and to the top of the index pointer 25*45 centimetres ( = 10*0 

 inches) ; every movement horizontally, therefore, of the loop M is 

 attended by a movement, twenty times the amount, of the top of the 

 pointer. Q is a screw for supporting the axis N. I have found it 

 convenient to put the zero-figure of the index towards the left-hand 

 side of the index-plate. R is a separate piece of wood fitting into a 

 rectangular hole in the base-board ; it carries a graduated rule, S, for 

 measuring the length of the wire to be heated, and is easily removed, 

 so that the wire may, if necessary, be heated by means of a row of 

 Bun3en's burners. The rule T is used when measuring the amount 

 of strain. U is a vertical stud or pin of brass (of which there are 

 two) for limiting the range of movement of the pointer 0. 



In using this apparatus, a straight wire or ribbon, H, of a suitable 

 length and thickness was inserted, the index pointer brought to by 

 adjustment of the sliding wire G, and a suitable amount of strain 

 (varying from less than two ounces to upwards of twenty) put upon 

 the wire by adjusting the sliding hooked wire E. One pole of a vol- 

 taic battery, generally consisting of six Grove's elements, was con- 

 nected with the binding-screw C, and the other pole then inserted in 

 the mercury-cup of B. As soon as the needle O attained a maximum 

 or stationary amount of deflection, the battery-wire was suddenly re- 

 moved from B, and the wire allowed to cool. The movement of the 

 needle was carefully watched both during its movement to the 

 right hand and also during its return, to see if any irregularity of 

 motion occurred. 



Wires of the following metals and alloys were employed : — palla- 

 dium, platinum, gold, silver, copper, iron, lead, tin, cadmium, zinc, 

 brass, german-silver, aluminium, and magnesium ; metallic ribbon was 

 also employed in certain cases. 



In these experiments the thickness and length of the conducting- 

 wire or ribbon had to be carefully proportioned to the quantity and 

 electromotive power of the current, so as to produce in the first ex- 

 periments with each metal only a very moderate amount of heat ; 

 thinner (and sometimes also shorter) wires were then successively 

 used, so as ultimately to develope sufficient heat to make the metal 

 closely approach its softening or fusion-point. The battery employed 

 consisted in each case of six Grove's cells, each cell containing two 

 zinc plates 3| inches wide, and a platinum plate 3 inches wide, each 

 immersed about 5 inches in their respective liquids. The amount 

 of tension imparted by the elastic band required to be carefully ad- 



