304 
The Electric Light for India . 
[April, 
eledtro-magnet should move i m.m. by one whole turn. In 
order to know the adjustment at any one time, and to be 
able to make the same adjustment again, a millimetre scale 
is to be attached, by which the movements of the screw can 
be read. The circumference of the head of the screw is to 
be divided into ten equal parts. Hence by this arrangement 
the comparative tensions of the spring or springs can be 
read up to o*i m.m. This micrometer screw, after the lamp 
has been regulated for any given current varying within two 
known limits, is to be fixed by a clamp and screw to keep 
that adjustment constant. 
4. The distance between the eledtro-magnet and its arma- 
ture is also to be made adjustable by a micrometer screw, 
provided again with millimetre scale like the above, and 
with a clamp and screw, for making any best adjustment 
constant . 
5. The clockwork and all regulating parts of the lamp to 
be entirely covered by a strong metal case, which is to be 
construdted in such a manner that it can be taken off or put 
on without interfering in any way with the adjustment of 
the lamp. The case may be cylindrical, opening like a door 
in two halves on hinges, and with a key for closing it. None 
of the adjustable parts of the lamp are to protrude, as it is 
intended not to touch the lamp after its proper adjustment, 
which is done in the laboratory only. Each lamp has only 
one best adjustment for any given current varying between 
two known limits, and the best adjustment is made constant 
by fixing everything. The case or cover is then closed, and 
the lamp put up for use. 
6. The eledtro-magnet which pulls the arc should offer no 
more resistance than 0*02 S.U. It is to be shunted by 
another eledtro-magnet which offers exadtly the same resist- 
ance. Hence one-half of the current passes through the 
eledtro-magnet of the lamp, and is made use of for pulling 
the arc. The amount of iron used in the eledtro-magnet, 
and the number of convolutions, should be such that at the 
mean distance of the eledtro-magnet from its armature the 
magnetic force is strong enough to produce an arc of 2*5 m.m. 
against the mean tension of the spring or springs when 
employing a current of about 25 Webers. The magnetic 
adtion of the shunt for the same current should be about 
double that of the eledtro-magnet of the lamp, in order to 
leave a margin for a finer adjustment, i.e. 9 equalisation of 
the magnetic adtion of the two. The two eledtro-magnets, 
each forming a shunt to the other, are adjusted in such a 
manner that the extra currents they produce, when the 
