i) They should be non-magnetic.
ii) They should be proof from mechanical fatigue.
iii) Where the springs are used to lead current into moving system they should have a small resistance, their cross-sectional area must be sufficient to carry the current without a temperature rise which effects their constants. They should also have a low resistance temperature co-efficient. Material A number of non-magnetic materials like silicon bronze hard rolled silver or copper. platinum, silver, platinum-iridium and German silver have been used but have not been found satisfactory due to some reason or the other. For most applications phosphor bronte has been the most suitable material except in instruments or low resistance (like milli voltmeters). In this case some special bronze alloys having low resistance may be used with some sacrifice in mechanical quality. Since deflection 0 is directly proportional to current I, the spring — controlled instruments have a uniform or equally spaced scales over the whole of their range.
To ensure that controlling torque is proportional to the angle of deflection, the spring, should have a fairly large number of turns so the angular deformation per unit length on full-scale deflection, is small. Moreover, the stress in the spring should be restricted to such a value that it does not produce a permanent set in it. Fatigue in springs may be avoided to a great extent by proper annealing and ageing during manufacture.
In order to eliminate the effect of temperature variations upon the length of the spring, two springs coiled in opposite direction are used. When the moving system deflects, one spring is extended while the other is compressed.
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