454 Prof. R. Threlfall on the Measurement 



screws are conical, and the distance from the end of the thread 

 to the point of the screw is 5 centim. The screws are of 

 steel tempered to the blue, the heads of brass ; and the tap 

 used to produce a thread in the holes through the brass plate 

 was identical with one of the screws ; the lathe being set to 

 use the same part of its screw and of its change wheels 

 during the making of each ; the measurement showed that 

 the screws were very good especially in the middle portion ; 

 they were also practically exactly alike. The goodness of the fit 

 in the brass plate was shown by the fact that an increase of 

 temperature of 20° C. was sufficient to " bind" the screws 

 very perceptibly. Indices similar to the indices of spherome- 

 ters were erected in the upper plate of the apparatus — one 

 index for each screw. All the workmanship being accom- 

 plished, the plates were next platinated by a process given in 

 Gore's i Electro-metallurgy ' under the name of " Roseleur's 

 Process;" a previous experiment showed that when the 

 directions are faithfully carried out, this process will yield a 

 hard bright deposit of platinum. The bath, however, is very 

 troublesome to keep in order since no solution of platinum 

 takes place to supply the place of that deposited. To prepare 

 the plates for platinating, they were first heated to the tem- 

 perature of boiling water and rubbed on the scraped surfaces 

 with a solution of caustic potash. Finally, they were rubbed 

 with a bit of fine pumice dipped in dilute caustic potash. 

 This is by far the best laboratory method known to me for 

 preparing surfaces of brass for electro-plating ; a clean 

 surface is obtained with comparatively little abrasion. Before 

 I found this out, I was much troubled to secure a good deposit. 

 After platinating, care being of course taken to prevent the 

 deposit being unequal, the plates were carefully washed and 

 dried. It was noticed that the metallic surfaces were covered 

 with a faint bloom of black platinum. On placing the plates 

 together and moving the top one slightly, the lower plate at 

 once adhered, and though weighing several pounds could 

 easily be lifted by the upper one. On pulling the plates apart 

 the " bloom" was found to be burnished practically all over 

 both surfaces, showing of course that the platinating had not 

 sensibly altered the planeness of the surfaces. For this 

 accuracy I am much indebted to the university assistant, 

 Mr. James Cook, who, being accustomed to prepare optically 

 flat surfaces, was led by the application of experience gained 

 in that way to the happy result above mentioned. 



The exact position of the two plates with respect to one 

 another was secured by cementing the plated surfaces to- 

 gether with hard paraffin. Two lugs of thick brass were 



