536 Farwell and Stijler — Common Laboratory Experiments. 



This apparatus may readily be converted into a fairly satisfac- 

 tory hot wire ammeter, merely by connecting the ends of the 

 wire to the circuit. A desirable feature is the addition of a 

 heavy metal tube to enclose the wire. 



As set up for purposes of demonstration in our laboratory 

 the apparatus consisted of a piece of No. 30 B. & S. brass wire 

 about 130 cm long, connected to the circuit by means of the 

 support at the top, and at the bottom through a mercury cup 



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15 



20 25 30 35 



Micrometer microscope readings 



40 



in which hung a small weight attached to the wire. The 

 weight served to keep the wire straight and under uniform 

 tension, while the mercury served for the double purpose of 

 making contact and damping any pendulum motion. The 

 wire was surrounded for its whole length by a heavy piece of 

 gas pipe, closed at the top. The increase in length was 

 observed by a micrometer microscope. The curve above gives 

 the results of one set of observations taken by one of our stu- 

 dents, the current range being from *15 to 1*12 amperes. 



Observation of the time required for the wire to return to 

 its original length would make the method available for the 

 determination of the mean coefficient of linear expansion of 

 metal in the form of wire, though probably not with any 

 degree of accuracy. 



