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 » Rheostat (0399)

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DESCRIPTION

Exhibit 399 is a rheostat, a variable electric resistance; its value depends on the position of the slider that moves along the winding. The maximum value is 4.5 Ω. See exhibit 111.

This type of rheostat can be used as a shunt to measure the resistance of a galvanometer using the method of half deflection as illustrated in design-2. Upon closing the switch (Formula: [\rm K_1]), and leaving open (Formula: [\rm K_2]), the current (Formula: [\rm I_g]) that flows in the galvanometer is


Equazione: [\rm I_g = \frac{E}{R+G} \eqno{(i)}]

Where,
R = known resistance inserted in the circuit
G = unknown resistance of the galvanometer
E = d.d.p.

If (θ) is the rotation angle of the index of the galvanometer one has:

Equazione: [\rm \frac{E}{R + G} = k\theta]

where k is a constant of the galvanometer

Now closing switch Formula: [\rm K_2], one chooses a resistance from the adjustable rheostat (shunt) (S) such that the deflection becomes half (Formula: [\frac{\theta}{2}]), then(Formula: [\rm I_g]) is given by:


Equazione: [\rm I_g = \frac{E}{R+\frac{GS}{G+S}}\cdot\frac{S}{G+S} =]


Equazione: [= \frac{ES}{R(G+S) + GS} = k \frac{\theta}{2}\quad\quad\eqno{(ii)}]

From (i) e (ii):


Equazione: [\rm G = \frac{RS}{R-S}]

For R much greater than S one has


Equazione: [\rm R - S \approx R]

Thus

Equazione: [\rm G = S]

ISCRIZIONI

  1. Su una etichetta: Ing. S.Belotti & C. S.A. Milano
    tipo 195 n.o 20991
    Ohm 4,5 A. 2,25
    Carico -------

Dati Catalografici

Data di costruzione:---
Data di carico:1937-8-5
Nr. Inventario:4858 (Generale dal 1874)
Costruttore:Belotti
Materiale:metallo, mica
Dimensioni:Lunghezza complessiva: 27 cm;
Larghezza: 10 cm
Conservazione:cattivo (manca l'avvolgimento)