RESEARCHES IN MAGNETISM. 
529 
general much more homogeneous. Their small diameter also permits a solenoid of 
many turns to be wound on them without introducing much resistance, so that a 
strong magnetising force is obtainable without difficulty. With the single exception 
of an observation of cast-iron (§24 below), all the experiments to be described were 
made with wires. In some cases the wire has been bent into a ring and welded to 
form an endless magnet, but generally I have used long straight wires, which, as 
regards convenience of winding and other particulars, have many practical advantages 
over rings. A good form of magnet, especially appropriate in the examination of 
cast-iron and cobalt (where very long rods are impracticable), would be a nearly 
closed ring, with an opening between the ends just wide enough to allow a short 
induction coil to be slipped off. So far as the magnetisation of the middle portion 
was concerned, this would be nearly equivalent to a closed ring, while it would enable 
the actual magnetic state, and not sudden changes merely, to be measured. 
§ 8. Ballistic Method of Experiment .—In the ballistic method the changes produced 
by sudden changes of <£) were determined by the throw of the needle of a galvano¬ 
meter in circuit with an induction coil which was wound over the middle part of the 
rod, or over the whole or any convenient part of the ring. The galvanometer was a 
short coil Thomson’s, made for lecture purposes, and consequently with an unusually 
heavy mirror. Its period was moderately long, and the retardation, although sufficient 
to bring the needle to rest without special appliances, in the interval between 
observations, was not so great as to unfit it for ballistic use. To test its accuracy in 
this kind of work a preliminary experiment was made, in which a wire in circuit with 
the galvanometer was wound into a coil of ten turns, which was slipped over the 
end of a long permanent bar-magnet. Headings were taken of the throws given when 
this coil was suddenly drawn off the bar, and again, at each stage, while the number 
of turns in the coil was progressively reduced to one. The throws were found to be 
proportional to the number of turns, to a degree of accuracy as great as was 
attainable in the actual observations, and throughout the whole range of the scale. 
To reduce the ballistic readings to absolute measure, I adopted the method used by 
Rowland.'"' A coil of ten turns of thick wire, which will be called the earth-coil, 
having a total area of 10X1216 square centimetres, was wound on a light rectangular 
wooden frame, and was kept always in circuit with the ballistic galvanometer and 
the induction coil. It was laid flat on a horizontal table, and at the end of each 
experiment it was quickly turned over, thereby cutting twice the vertical component 
of the earth’s magnetic force, and the throw of the galvanometer was observed. This 
“ earth-coil reading” was the throw corresponding to 2x 12160X 0'34 lines of induc¬ 
tion, 0’34 being the assumed local valuef of the vertical component of the earth’s 
force. 
§ 9. Magnetising force was given by the current from a battery of gravity Daniell 
* Loc. cit., p. 148. 
f tn the Physical Laboratory of the University of Tokio, Japan, where the experiments were made. 
