ON TIII: Monru's OF TOKSIOXAI. KIOIPTTY OF METAL \vn;i:s \:\ 



runs MIII with a rush, leaving onlv a thin layer of water alnive tin- surface of the 

 mercury. Tliis is repeated, and tin- intermittent rush of water is ipiitc eHectual in 

 keepiug tlie surface of the mercury clean. The bouncing of the lever in hitting 

 against the stops was another difficulty which caused double sparking. This was 

 overcome hy covering the stops with cork. 



In order that the platinum wire at the end of the pendulum should make good 

 contact with the mercury, it was occasionally removed, scraped, heated, and amal- 

 gamated with sodium amalgam. It was then gently heated again to unamalgamate 

 it. This method recommended by C. V. BOYS* was found to give the best results. 

 The wire E on the contact breaker had to be periodically taken out and ground 

 smooth on an oil stone, for the sparking made the end ragged. It was then heated 

 and treated in the manner described above. 



In order to prevent sparking at the clock break, a battery of 10 small cells 

 (connected up in sets of two in series and the sets joined up in parallel), consisting of 

 strips of platinum foil in accumulator acid, were placed in parallel across it. These 

 soon l)ecome polarised, and their back E.M.F. prevents the current from the accumu- 

 lators from going through them, but the extra E.M.F. when the clock circuit is 

 broken sends a momentary current through these cells instead of sparking across tin- 

 air gap. 



Several other methods of obtaining second signals were tried with different kinds 

 of clock contact and various forms of relay, but after giving much trouble they were 

 discarded for the present arrangement, which has proved very satisfactory. 



I had expected that the stream of mercury would affect the rate of the clock, but 

 on rating it for several days with the mercury running continuously from 10 A.M. till 

 in P.M., no difference in the rate could l>e detected. The clock rate was determined 

 daily by comparing the time with that indicated by the clock in the Cambridge 

 Philosophical Society's Library. This latter clock keeps very good time, and is 

 checked once a week by a man from the observatory. Its weekly rate is alxmt 

 2 seconds. Twice a day at 10 A.M. and 6 P.M. a chronometer was carried across 

 to the library and its time compared with that of the Philosophical Society's clock, 

 and then immediately afterwards with the experimental clock in the Cavendish 

 Laboratory. The rates thus obtained were after wards corrected on account of the 

 rate of the library clock. The clock used in these experiments was found to keep a 

 constant rate for a week, altering slightly each time it was wound up. 



The vacuum tulie Diving the second signals was fixed vertically at a distance of 

 about 4 metres from the mirrors. The light from it was focussed by means of a glass 

 cylinder full of water on to a slit made by cutting a fine line on the back of a silvered 

 mirror. The images of the slit in the mirrors were viewed by a telescope placed 

 80 centims. from the minors, the slit and telescope being so arranged that tin- 

 incident and reflected light were nearly normal to the mirrors. 



'Phil. Trans.,' A, vol. 186, p. 25. 



