Physics · Ch 3 — Magnetism and Magnetic Effects of Electric Current
Moving Coil Galvanometer
Moving Coil Galvanometer
A moving coil galvanometer detects small currents using the torque of §3.11.1. Construction: a rectangular coil of many turns of insulated copper wire, wound on a light frame with a fixed cylindrical soft-iron core inside it, is suspended between the curved pole pieces of a horseshoe magnet (which produce a radial field), with a fine phosphor-bronze suspension strip (carrying a small mirror, for a lamp-and-scale reading) at the top and a phosphor-bronze hair-spring at the bottom. Working: because the field is radial, the coil's two active sides stay perpendicular to at every angle of rotation, so the deflecting torque is simply at all deflections (not , since is effectively fixed at by the radial-field design). This deflecting torque twists the suspension, developing a restoring torque ( = restoring couple per unit twist); at equilibrium,
where is the galvanometer constant. The figure of merit is the current needed for one scale-division deflection; the instrument is sensitive if a small current/voltage produces a large deflection. Current sensitivity increases with more turns , larger field , larger coil area , or a suspension with smaller (phosphor-bronze is used precisely because its is very small). Voltage sensitivity ( = galvanometer's own resistance). …
What this figure shows. A rectangular coil of many turns of insulated copper wire, wound on a light metallic frame with a fixed cylindrical soft-iron core inside it, is suspended between the curved (hemispherical) pole pieces of a permanent horseshoe magnet. A fine suspension strip carries current in and holds a small plane mirror, which reflects a lamp's beam onto a translucent scale so that the coil's tiny rotation shows up as a large, easily read movem …
What this figure shows. The rectangular coil PQRS of the galvanometer sits between the curved pole pieces of the horseshoe magnet, which produce a radial field so that the two sides QR and SP always stay parallel to B (feeling no force) while the two sides PQ and RS stay perpendicular to B at every angle of rotation, each feeling a force of the same magnitude BIl -- one drawn pushing upward and the other pushing downward, producing a steady deflecting torque regardless of ho …
Worked out. For a galvanometer with N=5, A=2x10^-2 m^2, B=4x10^-2 Wb/m^2 and torsional constant K=4x10^-9 N m/degree: (a) current sensitivity I_S = NAB/K = (5x2x10^-2x4x10^-2)/(4x10^-9) = 10^6 divisions per ampere, i.e. 1 division per microampere. (b) if 50 divisions correspond to 25 mV full-scale, the voltage sensitivity V_S = 50 div/25 mV = 2x10^3 divisions per volt. (c) the galvanometer's own resistance follows from R_g = I_S/V_S = (10^6 div/A)/(2x10^3 div/V) = 0.5x10^3 ohm …
Worked out. If a galvanometer's resistance is doubled (R_g to 2R_g) in order to raise its current sensitivity by 50% (I_S to 1.5 I_S), the new voltage sensitivity is V_S' = I_S'/R_g' = (1.5 I_S)/(2 R_g) = 0.75 (I_S/R_g) = 0.75 V_S, i.e. the voltage sensitivity actually falls by 25% even though current sensitivity rose -- a reminder that boosting one sensitivity by changing R_g does not automatically boost the other in the …