a Simple Rheostat. 175 



and the cylindrical part is amaloamated. This electrode is 

 connected by a short piece of Jie.cilde wire with a terminal 

 screw T'. 



The object of making the effective part of the platinoid 

 wire to enter the U-tube by the ru/ht limb and the copper 

 electrode to dip into the left limb, is to avoid the accidental 

 contacts between the platinoid wire and the part of the copper 

 electrode above the surface of the mercury, which I found 

 occurred when the electrode dipped into the right limb. 



The radius of the wheel is somewhat greater than the 

 radius of the bend of the U-tube, in order that the wire may 

 not be in contact with the glass at the level of the surface of 

 the mercury. 



Since the joint in the endless band is thicker than the rest 

 of the band, it is necessary, in order to allow the joint to 

 pass easily over the wheel, to give the U-tube a little play in 

 a vertical direction. The tube, therefore, slides quite freely 

 through some simple wooden guides, and the requisite tension 

 is given to the band by a plunger, which presses upon the 

 bend of the U-tube and is actuated by a small coiled spring. 

 The tube can thus rise slightly when the joint passes over the 

 wheels and the wire is always kept sufficiently tight and is 

 never over-strained. 



A rim 2J cms. high round the base board forms a trough, 

 which catches the few small globules of mercury which are 

 carried up by the wire and are sometimes shaken oif it. 



The resistance of the rheostat in a given position of the 

 wire is very definite, and violent tapping or shaking has no 

 effect upon it. 



As mercury attacks solder, care must be taken not to wind 

 the joint into the mercury. I find that the mercury also 

 attacks german-silver so rapidly that a german-silver wire 

 completely loses its strength after a few minutes' immersion 

 in the mercury. On the other hand, so far as I can judge 

 by three months^ experience, platinoid wire is quite unaffected 

 by the mercury. 



Since the wire must be quite flexible if it is to draw easily 

 round the bend of the U-tube, its thickness is limited, and 

 hence this type of rheostat is not adapted for use with large 

 currents. Yet a platinoid wire '015 inch in diameter will 

 carry 2 amperes without undue heating, and the heating- 

 leads to no inconvenience, for the wire, being exposed to the 

 air, takes up almost instantly a steady temperature corre- 

 sponding to the current passing through it. 



The figure represents a specimen of the rheostat made for 

 me by Messrs. W. G. Pye and Co., to whom I am indebted 

 for some help in the details of the design. 



