can employ with all his instruments an induction coil for installations
where the resistance of the line wire makes it desirable to do so; the
microphone and battery being included in the primary circuit and the
telephones in the secondary.

[Illustration: FIG. 3.]

Fig. 3 is an ingenious arrangement devised by Mr. Munro, in which the
adjusting spring or weight is substituted by a magnet which may be
either a permanent or an electro-magnet. The figure shows an arrangement
in which the fixed gauze, g, is perforated as in the apparatus
illustrated in Fig. 2, and the movable electrode, g, is bent or dished
so as to press upon g around its edge. E is a magnet which by its
attractive influence upon g holds t up against g with a pressure
dependent upon its magnetic intensity and upon its distance from the
gauze. By making E an electro-magnet, and including its coil in the
telephonic circuit, an instrument may be constructed in which the normal
pressure between the electrodes can be automatically adjusted to the
strength of the current, and in cases where an induction coil is
employed the magnet, E, may be the core of such a coil.

[Illustration: FIG. 4.]

Fig. 4 illustrates an apparatus devised by Mr. Munro, and to which the
name thermo-microphone might be given, as it is a microphone in which
thermo-electric currents are employed in the place of voltaic currents,
its special feature of interest lying in the fact that the heated
junction of the thermo-electric couple is identical with the microphone
contacts of the two electrodes. In this very elegant experiment a piece
of iron wire gauze, G, is supported in a horizontal position by a light