Equipotential Lines of an Electric Current. 431 
trials which furnished this table were made, both these steel 
strips had been subjected, in the usual position, to a magnet- 
izing force of about 8500 intensity. It does not seem pro- 
bable that the results just recorded were seriously affected by 
this previous experience of the steel. 
This first magnetization was for the purpose of detecting, if 
possible, a permanent effect, such as had been observed a year 
before in a piece of tempered clock-spring, but had been vainly 
looked for in soft iron. 
No. 2 showed a permanent effect equal to about 14 per cent. 
of the temporary effect, a result agreeing very well with that 
obtained with the tempered clock-spring. No. 1 showed a 
permanent effect relatively, and even absolutely, larger than 
that in No. 2. It was about 5 per cent. of the temporary 
effect. In both cases the tests made were hasty, and the re- 
sults obtained are only approximately correct. I believe it 
has been observed that residual magnetization is, under certain 
conditions, greater in soft than in hard steel. 
Gold. 
The only experiment of importance made with gold was a 
test for permanent effect. The strip used was the one experi- 
mented with the year before, a description of which has 
_already been published (Philosophical Magazine for May 1883, 
p. 342). The permanent effect, if any was produced, must 
have been a very small part of one per cent. of the temporary 
effect. 
Cobalt. 
In the ‘ Philosophical Magazine’ for September 1831 I 
said :—‘‘ No thin strips of the metal [cobalt] being at hand, a 
slice was sawn from a small block of moderately pure cast 
cobalt and worked into the form of a cross. ‘To the extremity 
of each arm of this cross was soldered a thin strip of copper 
. for the purpose of making the electrical connections. 
The cross of cobalt with the copper strips attached was now 
fastened with hard cement | beeswax and rosin] to a strip of 
glass,” &. It was the same piece of cobalt, reduced now 
with file and emery-paper to a thickness of about ‘0062 cm., 
that served during this new series of experiments. The central 
portion of the cross was about 2 millim. square, and the arms, 
about 3 millim. long and 2 millim. wide, projected from it at 
right angles. 
The main object of the recent test was to determine the 
effect produced in cobalt by change of temperature. It is 
assumed that the intensity of ie magnetic field remained con- 
2G2 
