Fig. 57.
[Fig. 56 B] represents a case in which notice is given at two places. By insulating a key as shown, reply signalling can be carried on between the points at which the bells are placed. A special gravity Daniell modification ([§ 25]) is used for this class of work ([Fig. 57]): a narrow lead cylinder, about 2" in diameter, watertight except at the bottom, where it opens out into an inverted cone, the surface of which is pierced with holes. This stands immersed in dilute sulphuric acid. A saturated solution of copper sulphate is next carefully introduced, so as to displace the acid upwards. Crystals of sulphate of
copper are introduced into the open end at the top of cylinder, to fill the perforated portion at the bottom. From the wooden cover of cell a thick flat ring of amalgamated zinc hangs suspended in the dilute acid. Care should be taken not to introduce the zinc till the two solutions have become well separated. During action this becomes coppered, while in contact with the sulphate of copper, but it is not attacked by the acid. It is, however, preferable to paint that portion of the lead, which is surrounded by the acid. The height of the cell is about 14."
It will be readily understood that if this latter system be employed, special contacts, which break contact when the pressure is removed, must be employed for the door or window contacts. A simple form is shown at [Fig. 58].
Contacts similar to Figs. [50], [53] and [54], may be fitted on tills or drawers.
Fig. 58.