Kxperimenters and learners of telegraphy often wish to use 'pen-circuit cells on short lines, but find that they are unable to call other stations without closing (keeping closed) their own key. But when the keys are closed the cells are polarized, and are soon worthless. The simple arrangement shown in the accompanying diagram will obviate all the trouble, enabling them to call other stations, and at the same time the keys are always open; the cells being used only when the instruments are being operated. Any number of additional instruments may be put on by following the method indicated in the diagram.

Fig. 231 - An open-circuit telegraph system.