294 Messrs. Ayrton and Perry on the Determination 



the Morse instrument, B the battery, P the paper, S the 

 seconds-pendulum, L the long pendulum, C, C the mercury- 

 cups, R a resistance small compared with that of M plus that 

 of the coil r, but large compared with that of M alone. The 

 whole constituted what is known as a " break-circuit chrono- 

 graph;" that is, a continuous ink mark was made on the paper 

 run out by clockwork, broken by a very small gap each time 

 the wire attached to the bob of the seconds-pendulum passed 

 through the mercury. These breaks, then, in the ink line in- 

 dicated seconds ; if, however, both pendulums were simulta- 

 neously in the vertical line, no break was made. Hence the 

 absence of a break in the line at the end of any special second 

 indicated coincidence of the two pendulums ; and in this way 

 the times of a large number of coincidences could be automa- 

 tically registered. 



During this set of experiments we could not measure the 

 length of the long fine steel wire with as much accuracy as 

 was desired, since, although we had two or three brass scales, 

 the makers had omitted to record on them at what tempera- 

 tures they were correct. However, assuming that one of them 

 was accurate at 0° C, then a rather large number of experi- 

 ments gave, as the value of g, 978*8 centimetres per second as 

 a first rough approximation. 



Subsequently we obtained from the Finance Department of 

 Japan the loan of two very beautiful standard brass scales, by 

 Deleuil of Paris, and guaranteed correct at 0° C. One was 

 graduated in millimetres ; the other consisted of a brass rod 

 with two pieces at its ends at right angles to the rod, and the 

 distance between the two planes of the inner surfaces of the 

 pieces was exactly a metre at 0° C. We now had, then, the 

 means of making a far more complete series of experiments 

 than before ; but as our trial pendulum was nearly ten times 

 as long as the seconds-pendulum of our clock, the method of 

 coincidences was an inconvenient one ; and so we merely 

 adopted the following : — The long pendulum alone controlled 

 the " break-circuit chronograph;" so that the number of breaks 

 in the line during any time indicated the number of vibrations 

 of the long pendulum in that time. At the commencement of 

 the experiment, after the pendulum had been set swinging 

 and the paper was running out at a fairly uniform speed, a 

 mark was made on it by tapping sharply the armature up with 

 the finger when a chronometer, lying beside the Morse instru- 

 ment, indicated a certain time ; and after an hour or so, the 

 paper being kept running all the time, a second mark was 

 sharply made on the paper when the chronometer indicated a 

 certain other noted time. So much paper had then run out 



