174 



Mr. G. F. C. Searle 



on 



is made up mainly of a piece of platinoid wire '015 inch 

 (•038 cm.) in diameter. One end of the platinoid wire is 

 soldered to one end of a piece 

 of thin " flexible '"' stranded 

 wire L, while at the other end 

 of the platinoid wire a small 

 ^ye is formed. To complete 

 the band, one end of a piece 

 of silk cord is lashed to the 

 flexible wire near the soldered 

 joint, and the other end of the 

 cord is tied to the eye at the 

 end of the platinoid wire. By 

 means of the device described 

 below, the endless band is kept 

 tio-ht enough to cause the 

 wheel to grip the band snf- 

 ficientlv firmly to ensure that 

 the band shall not slip in the 

 groove when the wheel is 

 turned. Then, by turning the 

 wheel, the length of the plati- 

 noid wire included between the 

 surface of the mercury in the 

 right limb of the U-tube and 

 the end of the flexible wire 

 can be varied from practically 

 zero to about 165 cms., the 

 consequent change in resistance 

 beino- about 6 ohms. The other 

 end of the flexible wire L is 

 attached to a terminal screw 

 T, and L is long enough to 

 enable tbsT joint to pass freely 

 over the wheel. 



A second metallic connexion 

 with the platinoid wire is ob- 

 tained by a copper electrode, 

 which dips into the mercury 

 in the left limb of the U-t^he. 

 This electrode is made in the 

 shape of a narrow strip of 

 sheet copper with a wider end, 

 like a spade. The wide part 

 is bent round so as to form a 

 nearly complete cylinder, which just fits into the glass tube. 



