Variable Mutual Inductances. 
157 
high accuracy. I have therefore designed a different and 
more elaborate arrangement which gives a much more 
extended scale. I proceed to describe the first model, which 
has been in use for a considerable time with most satisfactory 
results. 
The general arrangement of the apparatus is shown 
diagrammatically in figs. 1 and 2, which are plan and side 
Fiff. 2. 
£ 
view respectively. The primary* circuit consists of two- 
equal coaxial coils C and C, which are connected in series, 
their windings being in the same direction of turning. The 
secondary consists of the coils D and F in series. Of these 
coils, D is movable, being mounted on an excentric axis Q 
so as to be free to turn in a plane parallel to those of C and 
C and midway between them. Rigidly connected with the 
movable coil is a pointer H which moves over a circular 
scale of about 180° in extent and graduated to read directly. 
The coil F is subdivided into ten sections, which are in 
series, each of them being 0*1 millihenry, and their junctions 
are brought to a set of separate terminals or studs with a 
turning head. The range of the moving coil extends from 
— 0*002 to +0*11 millihenry. This gives a continuous range 
from up to 1 millihenry, readable near zero to 0*02 micro- 
henry, to 1 part in 500 at 0*1 millihenry, and to 1 in 5000 
at 1 millihenry. The subdivision of the coil F is easily 
carried out by the following artifice. The coil is wound 
with uniformly stranded wire of ten insulated strands, all 
the strands are connected in series, and the whole adjusted 
to give 1 millihenry f . If the stranding has been properly 
* It is only for convenience of description that the circuits are here 
distinguished as primary and secondary. They are really quite inter- 
changeable. 
t Mutual Inductances with stranded wires were used many years ago 
by Prof. Brillouin (Theses presentees a la Faculte des Sciences de Paris 
1882). 
