Section 27. Modifications of the Apparatus. 



3 



been obviated and the construction rendered much easier by making 

 the electrode entirely of a platinum-iridium alloy with 15 per cent iridium. 

 This material is about as hard as steel and takes a good, strong screw- 

 thread. Two forms of this electrode have been used. Of these, fig. 

 12 shows the form adapted to the inside of a quartz cup. It is made by 

 screwing two circular disks, one 0.5 mm. thick and 15 mm. in diameter, 



and the other 1.6 mm. thick and 6.3 mm. in diameter, 

 tightly onto a rod 3.17 mm. in diameter, and then 

 soldering them together and to the rod with pure gold. 

 The disks must be screwed on, since riveting with 

 subsequent gold soldering does not hold. The rod is 

 then held in the lathe-chuck while the upper portion is 

 turned true. 



For poorly conducting solutions the quartz cup may 

 advantageously be replaced by a plain cylinder, as this 

 exposes less quartz surface to the action of the solution. 

 The electrode is then given the form shown in fig. 13. 

 Fi B . 12. Fig. 13. As. before, the rod is screwed and gold-soldered into 

 the upper portion of the electrode. To increase its effective area this elec- 

 trode is corrugated both along and at right angles to its axis, and to facili- 

 tate the escape of gas and vapor bubbles the points resulting from the cross 

 corrugations are made to slant upwards, as shown in the figure. For the 

 same purpose a series of concentric grooves is turned in the upper end 

 surface. 



With the use of this platinum-iridium alloy instead of steel for the 

 electrode rod, the compensating washer, Z, fig. 1, has to be made of some 

 material having a smaller coefficient of expansion than steel. Fused 

 silica has been found entirely satisfactory for this purpose ; and, as it 

 is an electrical insulator, the mica washer M is no longer needed. Since 

 fused quartz is very brittle it is necessary to make the upper portion of 

 the nut iV larger, so as to present a larger bearing surface on the washer. 

 It is conveniently made from a piece of round brass rod, the lower portion 

 being filed hexagonal to take the wrench. 



It was thought that cups of fused silica might be cheaper to make 

 than those of quartz-crystal, and this has proved to be the case. But 

 the material is not adapted to this purpose, because of its extreme brittle- 

 ness, for much more care has to be exercised in working it and the little 

 ridges which serve to make the joints tight seem invariably to crack 

 upon using. 



