34 Proceedings of Royal Society of Edinburgh. [sess. 
central square, were slipped into four rectangular coils from 
opposite sides. The arms which thus overlapped were firmly 
clamped together. Each coil consisted of two independent wind- 
ings. The inner windings of the four coils were connected in 
series, and with the secondary terminals of a small induction coil. 
The outer windings were likewise connected in series, and with 
suitable resistances and a source of constant E.M.E. 
It is evident that connections could be arranged so that a 
unidirectional current in either winding would produce an induc- 
tion either co- directional or transverse, relative to the axis of the 
exploring coil. A rocker in each circuit conveniently effected 
this alteration. 
The constant E.M.E. connected through resistances with the 
outer windings was used, (1) to demagnetise the iron by reversals 
decreasing from a maximum, and (2) to produce the field (cyclic 
or otherwise) and the co-directional induction measured by the 
exploring coil. 
The induction coil in series with the inner windings supplied 
the oscillatory current which agitated the iron of the central 
squares, either in a co-directional or transverse direction in the 
sense indicated. Let this agitation be called either the co-direc- 
tional or the transverse oscillations, as the case may be (determined 
by the position of the rocker in circuit), or when this distinction 
is superfluous, simply the oscillations. 
This rotation of the direction in which the oscillations act 
through an angle of 90° throughout the two central squares 
involves no change in the ampere turns nor in the measured 
reluctance of the iron circuits. What it does change is precisely 
that, the effect of which it is desired to measure. 
Field Superposition. 
It is now absolutely necessary to discriminate between the 
order and manner in which oscillations and field are superposed 
the one upon the other. Two experimental methods were 
adopted : 
A. Oscillations were superposed upon constant field. 
B. A change of field was superposed upon oscillations per- 
manently acting. 
