This is a reed electrometer that derives from the quadrant electrometer; but there are only two quadrants and the needle is also substantially halved and has the characteristic form of a reed.
The reed is made of aluminium, its mass is just one milligram, it is attached to a very thin platinum wire (Wollaston wire) stretched vertically and elastically at both ends, and is balanced, on the side opposite the wire, by a miniscule counterweight. In this way, the barycentre of the reed-counterweight system falls on the wire and the device is almost completely independent of gravity and is quite resistant to bumps.
During transportation, the reed of the instrument does not need to be blocked and prior to use it need not be levelled; it also supplies a prompt response and is quite stable.
The working is like that of the quadrant electrometer and the possible connections are also the same.
By means of a micrometric screw with precision to a hundredth of a millimetre, the two quadrants can be brought closer to or taken farther away from the reed, thus increasing or decreasing the sensitivity of the instrument. When used with the auxiliary field, the sensitivity can be varied, within certain limits, also by varying the auxiliary tension.
The instrument can be mounted on a special tripod and in that case the reading of the deviations is performed (with the Poggendorff method) thanks to a small mirror attached to the wire from which the reed is suspended; it can also be mounted on a projection device, containing a projector and scale, and in that case the reading is made directly on the scale with the aid of a lens mounted on the window of the electrometer.
With a sensitivity of 1 mV/mm and the scale at a distance of 1 metre, the electrometer is quick to respond and quite stable.
When the instrument is mounted on the tripod (heavy alloy, painted black and with adjustable feet) its overall height is 20 cm. The projection device, composed of a projector and a scale with zero at the centre (graduated to 250 mm) is contained in a trapezoidal sheet steel case with three adjustable feet and is painted black.
The case, made of chrome-plated brass, is composed of two identical, vertical and coaxial cylinders (height 5 cm, diameter 2.7 cm) and of a third, horizontal cylinder (height 4.5 cm, diameter 4.6 cm) placed between the two vertical cylinders.
The horizontal cylinder contains the quadrants and the needle; the two vertical cylinders contain the platinum wire on which the needle is suspended, the ends of which are fixed at the centre of two agate insulating windows that seal the electrometer at top and bottom.
To one of the bases of the horizontal cylinder are attached:
1) the plugs for connections to the plates, which are insulated by small blocks of agate;
2) the plug for earthing the case;
3) a lever that allows connection of the two quadrants to each other and to the case.
On the lateral surface of the horizontal cylinder we find on one side, at the level of the mirror, a glass window and on the other side a small glass receptacle communicating with the inside and capable of containing a hygroscopic substance.
Perucca (1949), Vol. II, p. 542
Vaschetti e Masera, Elettrometro a palette del Prof. Perucca (depliant illustrativo)