Galvanometer for Alternate Current Circuits. 311 



stampings are of two kinds — a rectangular portion with two 

 straight limbs, a, b, forming the core of the electromagnet, an I 

 a specially shaped stamping, p, between the poles. The 

 stamping, p, is separated from the magnet limbs on each 

 side by an air-gap. Each gap consists of two portions, a 

 narrow part about 1 mm. across bounded by the straight 

 e:lo-es of p, and a wider portion in which one of the vertical 

 sides of the moving coil can turn round the curved edges 

 of p. The moving coil, of oO turns, is similar in shape, 

 suspension, and mode of control to that of an ordinary 

 permanent magnet instrument. It has a central spindle 

 indicated at s and working in a recess suitably stamped in p. 

 The pile of stampings is nbout 4 cm. deep and 9 cm. long, 

 the limbs being 1 cm. wide. Each of the limbs, a, 6, i-5 

 wound with a coil of 2000 turns of fine wire, and also with a 

 coil of 100 turns of thicker wire. The two fine wire coils 

 are put in series and connected to three terminals ; the two 

 thicker coils are also put in series and joined up to two 

 additional terminals. Thus the instrument can be excited by 

 a winding consisting of either 200, 2000, or 4000 turns, 

 according to the voltage used. The iron is not too strongly 

 magnetized if the winding used contains 20 turns per volt on 

 50 cycle circuits. Thus 200 volts may be applied to the 4000 

 turn coil, or 10 volts to the 200 turn coil. But the instru- 

 ment is so sensitive that such excitation will only be needed 

 for exceptional tests. The moving coil may be used with a 

 condenser or other apparatus, either on some special circuit, 

 or in conjunction with one or other of the field coil windings. 

 The instrument has been constructed by Robt. W. Paul, to 

 whom several of the working details are due. The following 

 are some of the uses : — 



Use as a Voltmeter. 



If a voltage V be applied to a field coil of m turns and if 

 another voltage Y l be applied through a condenser of K 

 microfarads to the moving coil, it can be shown that the 

 torque acting on the moving coil is a measure of 



Akw, ....... (2) 



or of the mean product of the two voltages. If the con- 

 denser voltage is obtained from one of the field coils of n 

 turns the torque is measured by 



K 



/V 2 



"(I)- ■ • • • • • • e> 



