Electrical Resistances of Metallic Wires. 435 



the wire. The different diameters and rigidities also deter- 

 mined the necessity of being able to vary the tension. The 

 insulation is obtained by using an ebonite drum to receive the 

 wire ; on this drum a right- and left-handed screw has been 

 cut. The variation in the tension may be obtained in two 

 different ways, or by a combination of both: — 1st, by altering 

 the load carried in the little waggon which runs up and down 

 the inclined plane ; 2nd, by altering the angle of inclination 

 of the plane ; or we may combine both if necessary. 



As the application of longitudinal strain to a ware while 

 being tested for resistance has been shown to be considerable, 

 it was necessary to relieve the wire from the strain due to the 

 load before taking the resistance, otherwise the changes due 

 to strain would have been included in the readings, To 

 remove the strain, the waggon on arriving at the bottom of 

 the plane rests against a stop ; on being drawn to the top the 

 plane is brought to a position slightly above the horizontal ; 

 these allowing in both cases the drum to slightly unwind itself, 

 and consequently the strain on the wire is almost entirely 

 removed. 



The ebonite drum, as before observed, has a right- and left- 

 handed screw cut upon it ; these meet in ihe middle of its 

 length. A screw holding down an ebonite cap here passes 

 through the inside of the loop of the wire under test, and fixes 

 it firmly, but without injury, to the drum ; the ends of the wire 

 are carried up the plane to two binding -screws, and with the 

 necessary leading wires form one side of a Wheatstone bridge. 



A long length of stout whipcord is attached to the middle 

 of a winding drum underneath the head of the plane, and the 

 ends, after being coiled between the empty threads of the 

 ebonite drum (starting from the centre), are fixed to the ends 

 of the drum. On turning the drum below, the cords pull on 

 the top part of the ebonite drum, causing it to revolve; and at 

 the same time that the cord is being payed off, the wire under 

 test is being coiled on in its place. This is continued until 

 the waggon arrives at the top ; a pawl working on a ratchet- 

 wheel prevents the waggon running back until the plane has 

 been brought to a horizontal position, where, after resting five 

 minutes, the resistance is taken. The operations are now 

 reversed; and after a similar rest at the bottom, the resistance 

 is again taken. And these operations are repeated, each ope- 

 ration taking about six minutes, giving an average of ten per 

 hour ; they were generally continued for three hours, giving 

 for each series about thirty readings, and then allowed to stand 

 until the following evening, all strain being previously removed. 



The temperature of the laboratory was carefully regulated 

 each evening, for two hours before commencing work, to the 



