There are two types of controlling devices:
(i) Spring control
(ii) Gravity control.
i. Spring control. Fig. 1 shows a commonly used spring control arrangement. It utilises two spiral hair springs, 1 and 2, the inner ends of which are attached to the spindle S. The outer end of spring 2 is fixed while that of 1 is attached to a lever, the adjustment of which gives zero
adjustments. The two springs 1 and 2 are wound in opposite directions so that when the moving system is deflected, one spring win.ds up while the other unwinds, and the controlling torque is due to the combined torsions of the springs.
Since the torsional torque of a spiral spring is proportional to the angle of twist, the controlling torque (Tc) is directly proportional to the angular deflection of the pointer (θ).
Tc ∝ θ
The spring material should have the following properties:
(i) It should be non-magnetic.
(ii) It must be of low temperature co-efficient.
(iii) It should have low specific resistance.
(iv) It should not be subjected to fatigue.
i. Gravity control. With gravity control, weights Land M are attached to the spindle S [Fig. 2 (a)], the function of L being to balance the weight of the pointer P. Weight M therefore provides the controlling torque. When the pointer is at zero, M hangs vertically downwards. When P is deflected through angle θ , the controlling torque is equal to (weight of M × distance d) and is therefore proportional to the sine of the angular deflection [Fig. 2. (b)),
Tc ∝ sinθ
The degree of control is adjusted by screwing the weight up or down the carrying system.
It may be seen from Fig. 2 (b) that as e approaches 900, the distance 1-2 increases by a relatively small amount for a given change in the angle that when θ is just increasing from its zero value. Hence gravity-controlled instruments have scales which are not uniform but are cramped or crowded at their lower ends.
- The gravity controlled instrument is cheaper than corresponding spring-controlled instrument.
- It is not subjected to fatigue.
- It is unaffected by temperature.
- Gravity control gives a cramped scale.
- The instrument must be levelled before use.