Page 888 - Basic Electrical Engineering

capacitance of the wire to a very low value, which is desirable. The end
connecting leads are twisted to minimize any inductive effect.
When a strain gauge transducer is attached to the surface of a structure by

means of an elastic cement, any change in the surface dimension of the
structure due to stress formation will be reflected as some change in the

resistance value of the strain gauge. The sensitivity of a strain gauge is
expressed as gauge factor. Gauge factor is the change in resistance per unit

change in length of the strain wire. To make the transducer more sensitive,
the material for the wire should be so chosen that the gauge factor is high,

i.e., there is considerable change in resistance due to strain. Flexible silicon
strain gauge has a gauge factor much higher than a metallic gauge.
Two types of strain gauges are in use. They are bonded type and

semiconductor type. Bonded strain gauges are available in different shapes.
These are bonded with elastic cement to the surface whose stress is to be

measured. The bonded strain gauge consists of a wire grid whose shape may
be square, rectangular, or circular. The wire grid is on a base paper as shown

in Fig. 12.10 (a) and Fig. 12.10 (b)
A semiconductor strain gauge is made from a single piece of

semiconductor material. There is change in resistivity when the material is
strained. Many adhesives have been developed for pasting strain gauges to
specimen structures and surfaces. A semiconductor strain gauge has been

shown in Fig. 12.10 (c).