The moving parts of these instruments are usually mounted on pivots which are carefully finished to reduce the friction to a small value, and the instruments may be considered equiya lent to galvanometers arranged with the moving parts mounted on pivots, instead of being mounted upon a delicate suspension, and each provided with a pointer arranged to play over a scale graduated to read in amperes.
If the magnetic force caused by the current in the coils of an ammeter had nothing except the friction to overcome, every current capable of moving the pointer would pull it entirely across the scale. As the instrument should be constructed so that the range of movement of the pointer is proportional to the current in the windings, a proper force must be arranged to hold the pointer back, and this may be done by properly counter-weighting the parts or using a suitable spring to oppose the magnetic force set up by the current.
Instruments of the first class may be cheaply constructed, and formerly weze commonly made by dynamo builders for use in electric light plants, but it is difficult to make them extremely accurate because the coercive force of the iron prevents it from responding equally to equal magnetic changes. For this reason instruments of the first class cannot, as a rule, be used where great accuracy is essential, but only where an accuracy within from 2 per cent to 5 per cent is sufficient. For measurements that re quire greater accuracy, instruments belonging to the second or third class are usually used, and these can be made so that their readings do not vary more than one-half of 1 per cent from true values when the instruments are used with proper care.
The best form of such instruments consists of a modified D'Arsonval galvanometer with a movable coil mounted upon pivots and ar ranged with a pointer to play over a scale, which was first successfully produced by Dr. Ed ward Weston. The Weston ammeters and volta meters made of this construction may be prop erly said to have revolutionized the everyday measurements of amperes and volts.
Fig. 3 shows a plan of a Weston ammeter for measuring direct currents. A A represent the binding posts of the ammeter through which the current may be led to and from the instru ment. V' W are wires within the instrument, and E consists of a series of conducting shunts between the conductors W W. The movable coil C is connected by the wires w w with the binding posts, and through this movable coil there flows a fixed proportion of the current, which bears a ratio to the total current depend ing upon the electrical resistance of the shunt E and of the movable coil. The movable coil
is mounted on pivots within the magnetic field of the permanent magnet M, and any motion which is caused by the magnetic effect of a current flowing through the coil is opposed by the spiral spring D. The spring and the pole pieces of the magnet M are carefully designed so that the movement of the coil shall be directly pro portional to the current flowing through the coil, and the deflection is indicated on the scale S by the pointer B that is attached to the coil.
Fig. 4 shows a sectional end view of the working parts of one of these instruments. A stationary soft iron cylinder is mounted within the movable coil C for the purpose of produc ing radical direction and uniform density of the magnetic flux in the space between it and the pole pieces of the magnet, within •which space the conductors of the movable coil move. Weston or similar instruments are used a great deal where accurate portable current uring instruments are required, and instruments following this type are now manufactured in large numbers in this countries.
Magnetic instruments belonging to the third class are frequently called electrodynamometers because their indications are caused by the magnetic effect of the current in the fixed coils acting on the current in the movable coils.
Fig. 5 shows an early form of electrody namometer which is arranged for use as an am peremeter. This is often called the Siemens Electrodynamometer. The coil marked F is fastened to the frame of the instrument, and the coil marked M, which stands at right angles to the first, is suspended by a heavy silk fibre or a wire so that it is free to rotate. The ends of the conductor composing the movable coil dip into little cups C C containing mercury, and these are connected with a circuit arranged so that a current can enter and leave the mov able coil. The spring G is attached at one end of the movable coil, and at the other end it is connected to a thumbscrew T called a torsion head, by means of which this spring may be twisted. When a current flows in the coils, the magnetic force tends to turn the movable coil so as to place it parallel with the fixed coil.