410 Mr. H. Darwin on 
of these can be cut out at will by means of a mercury key. 
By setting over this key, the temperature at which the bridge 
is balanced is lowered 40° C. By the action of a similar 
key cutting out a coil in one of the copper arms, the tem~ 
perature of balance is raised 20°. The four combinations of 
the positions of these two keys give a range of 60° by steps 
of 20°. Intermediate settings are made by means of a 
compound slide-wire whose total effective range corresponds 
to about 22°. This compound slide-wire consists of the 
slide-wire proper with a range of about 7° and three coils 
with a range of 5° each. If these three coils are called 
a, b, c, respectively, while the slide-wire is called s, then we 
have a, b, c and s, always joined in series between the points 
B, B, in fig. 1, while a special switch, which can be set in 
four positions, enables the order of the resistances between 
these two points to be made a, b,c, s; b, c, s, a3 ¢, s, a,b: 
or s, a, b, c, at pleasure. 
The main circuit includes in series with the permanent 
heating-coil, some or all of the series-coils F. The current 
is led to the rail N, from which it passes, by way of the 
slider W, to one or other of the blocks L, according to the 
position of W. Thus it is evident from fig. 1 that the re- 
sistance offered by the series-coils included in the main circuit 
may vary, and the amount it varies is from 0 to 30 ohms. 
The galvanometer G is of the suspended coil type, a long 
boom G, being secured at right angles to the axis of the coil, 
so as to swing in a horizontal plane whenever the deflexion of 
the galvanometer varies. The connexions are so arranged, 
that when the temperature of the copper arms of the con- 
trolling-bridge B rises above its normal valne (assumed as 
above to be 25° C.), the galvanometer-boom is deflected 
“ upwards” in the diagram, a fall belowthe normal temperature 
causing a deflexion “downwards.” The sleeve X is kept 
steadily rotating on a fixed shaft S by means of a small 
worm-gear M driven from shafting. The direction of rotation 
is such that points above the axis are carried towards the 
reader in fig. 1. ThecamC performs a variety of functions ; 
it is rigidly connected to the sleeve X, and is thus capable of 
displacement endwise with respect to the fixed shaft S. 
A helically bounded cylindrical segment D (called the 
shunt-circuit drum) is rigidly connected with the cam C, 
so that these two pieces rotate with the sleeve X, while 
a contact-piece W (called the slider) agrees with it in 
endwise displacement. The rocking-shaft R carries the hit- 
or-miss arm H, and the shunt-cireuit brush J, both of which 
are made fast upon it, so that they always move together 
about the axis of the rocking-shaft R, while they are incapable 
‘an 
