» Thomson electrometer modified by Mascart (0023)

Immagini

DESCRIPTION

Exhibit 23 is a Thomson type quadrant electrometer modified by Mascart that employs a reference potential. In comparison to electrometers that do not employ a reference potential the Thomson type is more precise but the needle deflection changes direction according to the polarity of the reference potential and thus they cannot be used to measure alternating voltages. The quadrant electrometer can be made sensitive to tenths of millivolts.

The instrument is enclosed in a grounded brass case mounted on three adjustable screws for leveling.Inside the instrument there are two opposite quadrants of a hollow cylinder connected as shown in figure 2: A1 with A3 and A2 with A4. An 8-shaped needle is suspended by a thread of metalized silica glass and free to rotate in the hollow quadrants. In the absence of potential difference the needle lies symmetrically inside the quadrants. The readout of this instrument uses the Poggendorf method (exhibit 371).

In one application the positive pole of a battery is attached to A1-A3 and the negative pole to A2-A4 and its midpoint to ground as is the case. The unknown potential V is then applied to the needle so the deflection is toward A1-A3 if negative and vice versa.

In another configuration, one pole of the battery (say positive) is connected to the needle and the other pole is grounded as is the case as well as A1-A3. Then the unknown voltage is connected to A2-A4. If negative, the needle which is positive will rotate toward A2-A4, toward A1-A3 if positive.

For a precise operation the needle oscillations should be damped to record faster and more accurate reading.

A similar instrument is exhibit 352.

In the Cambridge Instruments catalog there is an electrometer apparently identical to this one, called â€œDolezalek Electrometerâ€ (see photo-1). A detailed description of an instrument of this type can be found in Perucca.

ISCRIZIONI

Sullo strumento, oltre al nome del costruttore (The Cambridge Scientific Instruments Co. Ltd., Cambridge, England) è inciso il numero di serie 3075.

BIBLIOGRAPHY

[1] The Cambridge Scientific Instrument Company, Ltd.,The Dolezalek Electrometer, England (Cambridge: 1906)
[2] URL [ http://www.sil.si.edu/digitalcollections/trade-literature/scientific-instruments/files/51618/ ]
[3] URL [ http://www.uniurb.it/PhysLab/strumenti/e24.html ]

Dati Catalografici