196 
DR. E. TAYLOR JONES ON THE RELATION BETWEEN 
All ballistic measurements were repeated a number of times, and mean values taken, 
to eliminate, as far as possible, small errors of observation. 
Measurement of Magnetic Change of Length. 
It was necessary to determine the change of length of the nickel wire under 
tensions of 2‘4 and 4’9 kilograms, for a series of increasing fields, the current being 
reversed a few times at each step. 
Instead of doing this directly, by first demagnetising the wire and then applying 
the current, it was found, more convenient to measure the temporary and residual 
changes of length separately. 
The temporary change of length was determined by first making and reversmg 
several times a measured current, and then observing the scale deflection caused 
by breaking the current. The reversed current was then made, the deflection 
observed on breaking it, and the mean of the two deflections taken. The wire 
always lengthened when the current was broken. 
No complete hysteresis loops were obtained, but the opportunity was taken of 
noting how much of the residual contraction could be removed by applying a 
reversed field. Thus each time the current was broken a small reversed current was 
applied, causing an increase of length, and this current was gradually increased 
until the length reached a maximum, and at higher fields again diminished. 
Next a series of residual contractions was determined, the wire being demagnetised 
by reversals before each reading and a current made for a short time, the scale 
reading being taken before the current was made and after it was broken. 
The residual contraction was always found to be greater than the contraction 
which can be removed by applying an increasing reversed current. This has also 
been observed by Mr. Nagaoka.* 
As the magnetising field is increased both the residual and the “ removable ” 
contraction at first increase and then become nearly constant, the former being then 
about 2'5 X 10“® and the latter 1‘9 X 10*^ of the length of the wire. 
The reversed field corresponding to minimum contraction was about 16 C.G.S. 
The temporary contraction was, however, still increasing in the highest fields 
employed, its value at H = 350 being about 33 X 10“*^, with a tension of 
4’9 kilograms. 
The curve representing the total contraction was obtained by adding the ordinates 
of the “ temporary ” and “ residual ” curves.! 
The difference between the tensions 2‘4 and 4*9 kilogs. weight was not sufficient 
* ‘ Phil. Mag.,’ Jan., 1894, p. 131. 
t Since about a centimetre of the nickel wire at each end was soldered in the brass pieces A and B, 
the length of wire whose changes were measured was only 83'2 centims. This exposed length Z was 
used in calculating a. 
