In a type built with vertical sections each division is complete in itself, and is not directly connected with the next section, but communicates with flow and return drums. A defective section may thus be left in position and stopped off by means of plugs from the drums until it is convenient to fit a new one in its place.
Fusible plugs are little used; they consist of pieces of softer metal inserted on the side of the boiler, which melt should the heat of the water rise above a certain temperature.
37); the opposed faces of these plugs are perfectly smooth, and are placed within a millimetre of each other.
Silver, sufficient to fill loosely about half the cavity between the plugs, which fit tightly into the tube.'
Marconi, by giving great attention to details, improved the electromagnetic tapper, and, combining it with his improved form of sensitive tube, made a telegraphic instrument as follows: the small glass tube, containing nickel and silver filings between two silver plugs, was attached to a bone holder, and under this was arranged a small electromagnet having a vibrating armature like an electric bell carrying on it a stem and hammer.
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