Construction of Pointers Electrical Indicating Instruments

Construction of Pointers Electrical Indicating Instruments Points : Construction of Pointers Electrical Indicating Instruments, Types of Pointers, Pointer Materials, Properties of Pointer Materials, Lightspot Indication, Use and Repair of Pointers A needle which deflects on a graduated scale to measure the quantity, directly by its deflection is called a pointer. The pointer of an instrument shows the position of moving system of that instrument.

Every indicating Instrument has a pointer. The shape and size of pointer used depend upon the type of instrument. In all cases, however, the weight and inertia of the pointer must be reduced as far as possible, both to reduce the load on the bearings of the moving system and to avoid the high degree of damping which would be necessary if the moving system had considerable inertia. Types of Pointers Shows a wide variety of shapes for the tip of the pointers.
(a) Beam-shaped pointer
(b) Spade-shaped pointer
(c) Spade-shaped pointer for edge-wise instruments
(d) Lance-shaped pointer
(e) Knife-edged pointer
(f) Hair-pointer (1 sheet of paper)
(g) Glass-pointer Pointer Materials As explained before that a pointer must be light in weight. For the sake of lightness, aluminium strip or tube is used for the pointer, a truss construction being used in some cases for rigidity.

In some instruments where precision in reading at close range, is aimed at, a strip of mirror is fitted on the plate which bears the scale. Properties of Pointer Materials Usually the pointers are made of Aluminium and Acryl materials. These have following properties 1) Light in weight
2) When exposed to air a thin layer of oxide is formed, on its surface which offers high resistance against Corrosion
3) Being soft materials the pointer long life
4) Due to malleable and ductile materials, the pointers can be made in any size and shape easily. These are cheap and easily available. Lightspot Indication The mass and inertia-free light beam can also serve as a pointer. The lightspot facilitates these of very light moving mechanisms with small moment of inertia. In place of the pointer, a mirror of 5-10 mm diameter or a rectangular strip with few mm square of reflecting surface mounted in the axial direction is used. The mirrors are finished with cut glass and often deposited with a layer of aluminium. They are also protected with a thin layer of quartz on the top. The mirror and scale arrangement doubles the effective angle of deflection. Use and Repair of Pointers The accuracy of reading of indicating instruments depends upon the condition of pointers. While using these instruments the pointers should not be misused otherwise the reading will be incorrect.

While repairing, the mechanical zero-point of the pointer should be adjusted properly by pointer zero- adjustment mechanism. Quite often, the fixed supporting point of the restoring spring, which lies next to the left side bearing, is rotated by means of a cam. The cam can be rotated from the outer side of the instrument by operating a screw-head provided for this purpose.