It is an instrument for measuring differences in potential of the type invented by the German physicist Ferdinand Braun (1850-1918), calibrated from 0 to 3500 volts (100 V/div).
It is completely screened by a cylindrical metal case painted black (diameter 20 cm, depth 8 cm) supported by a short metal staff on a black enamelled tripod
One of the feet has a screw that adjusts the indicator to zero prior to use.
The front and back of the case have windows giving a view of the scale and are covered with panes of glass allowing reading of the potentials.
The panes of glass and the underlying metal covers are held in place by a brass frame. At the top of the case the support of the needle and the graduated scale are attached to the top of the case by means of an agate insulating cap. The support of the needle is a flat, vertical metal rod which, bent twice at right angles, leaves a short horizontal part stretch.
The scale is attached to the bottom end of the shaft; the needle is free to rotate around a horizontal axis since it is housed in a longitudinal aperture of the horizontal part of the staff on the edges of which it lies on a pin passing through its barycentre.
The upper end of the shaft terminates on the outside of the case with a clamping pommel.
A vertical clamp is fixed to one of the feet of the instrument.
When the difference in potential to be measured is applied between the needle and the case, the needle is repulsed by the supporting staff and the value can be read directly on the scale.

Battelli - Cardani (1925), Vol. IV, P. I, p. 232
Murani (1906), Vol. II, p. 363

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