~Ques. What is the character of the scale and how should it be constructed?~
Ans. The scale is not uniform and should be hand made and calibrated under the conditions which it is to be used.
~Ques. What is the objection to moving iron ammeters?~
Ans. Since the coil carries the entire current they are large and expensive.
~Ques. What precaution should be taken in installing moving iron ammeters?~
Ans. Since gravity is the controlling force, the instrument should be carefully levelled.
~Ques. Describe an inclined coil instrument.~
Ans. It consists of a coil mounted at an angle to a shaft carrying the vane and pointer, as shown in fig. 2,498. A spring forms the controlling force and holds the pointer at zero when no current is flowing.
[Illustration: FIG. 2,498.--Inclined coil form of electromagnetic or moving iron instrument.]
~Ques. What is the principle of operation of the inclined coil instrument?~
Ans. When a current is passed through the coil, the iron tends to take up a position with its longest sides parallel to the lines of force, which results in the shaft being rotated and the pointer moved on the dial, the amount of movement depending upon the strength of the current in the coil.
~Ques. Describe a magnetic vane instrument.~
Ans. It consists of a small piece of soft iron or _vane_ mounted
[Illustration: FIG. 2,499.--Magnetic vane form of electromagnetic or moving iron instrument.]
[Illustration: FIG. 2,500.--Magnetic vane movement of a Wagner instrument; it is used both for voltmeters and ammeters. This type differs from the dynamometer movement in that a vane of very soft iron replaces the moving coil. The magnetic vane movement makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the needle to zero. The spring does not carry any current.] on a shaft that is pivoted a little off the center of a coil as shown in fig. 2,499, and carrying a pointer which moves over a scale.
Ans. Its principle of operation is that _a piece of soft iron placed in a magnetic field and free to move, will move into such position as to conduct the maximum number of lines of force_.
The current to be measured is passed around the coil, producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible.