VERDET’S CONSTANT IN ABSOLUTE UNITS. 
21 
not only to lend me a magnet, but also to have it vibrated at Kew, and to determine 
the temperature coefficient and the correction for torsion of the thread. 
The magnet was, at Pixholme, suspended to the same stone support and observed 
with the same telescope and scale as the smaller magnet used to determine the strength 
of the currents. It was therefore in exactly the same place as that magnet had been. 
The method used for taking the time of vibration was explained to the author by 
Prof. Maxwell, in a letter from which the following paragraphs are extracted * : — 
“ The best way to take the times is to begin with getting the approximate time of an 
odd number of half-vibrations 1, 3, or 5, according to convenience. Take such an odd 
number as will have a period of nearly half a minute or 20", or something easily 
reckoned, then note down the time of passage through a point near the point of rest, 
marking it + when it goes to right, and — when it goes to left, and observe, say, 
11 passages, 6+ and 5 — , at convenient intervals. Then after a time equal to that 
of 100 vibrations or near it, begin again and observe 11 passages. 
“From the two pairs of 11 passages find the time of vibration roughly, and thence 
the number of passages between the first and second set. Finally, find the time by 
dividing the mean interval between the first and second set by the number of passages. 
By taking the passages alternately -j- and — , you eliminate the error arising from 
taking the passage at a wrong point of the swing.” 
A large clock beating seconds was so placed that the observer at the telescope could 
see it by merely looking round. The times of passage Avere taken as follows : — 
The hour and minute having been written down by an assistant, the observer noted 
the second, and went on counting the beats by ear. Then looking into the telescope, 
he observed the time of every 7th passage, and called it out to be Avritten down by the 
assistant. The passages were thus always alternately + and — . The time occupied 
by 7 passages was sufficient to allow of the clock being looked at to get a fresh beat 
between each. 
The approximate times at Avhich each passage might be expected, within, say, 3", 
were previously written out and placed in a convenient position. This prevented the 
possibility of a 5th or 9th passage being observed instead of a 7th. A thermometer 
hung in the case (wood with glass front) in which the magnet SAVung gave the tempe- 
rature. 
The following Tables give the results of the experiments — that of “ Vibrations at 
Kew” being sent by Mr. Whipple, and those of “Vibrations at Pixholme” being 
determined by the author. 
* See also Maxwell’s ‘ Electricity,’ Art. 456, vol. ii. p. 102. 
