Construction of Bi-metallic Instruments

Construction of Bi-metallic Instruments Points : Construction of Bi-metallic Instruments, Damping of Bi-metallic Instruments, Properties of Bi-metallic Instruments, Application of Bi-metallic Instruments, Materials of Bi-metallic Instruments Two strips of metals of different coefficients of expansion are hot-rolled to form a hi-metallic spiral. A current flowing through this spiral causes an expansion of the inner end and hence a movement of the axis and the pointer. The angular deflection of the pointer is approximately proportional to the Square of the current. The instrument can therefore be used on both d.c. and a.c..

Is a bi-metallic spiral; through its fixed end the current to be measured is led in. The current is taken out through the copper strip “3”, which exerts compensation of ambient temperature variations. It carries no current insulating disc “4” prevents heat radiation on spiral “2” from the current carrying spiral. Damping No special damping arrangement is needed. The control torque is produced by the spiral itself. Properties The thermal inertia is very high and the time for attaining steady state deflection may be up to even “15” minutes. The accuracy obtainable is about 2.5%. These withstand high overload. They are not affected by stray fields. Frequency errors are noticeable only in the audio range and above. Application Bi-metallic instruments indicate the average current over a time interval of 8 to 15 minutes. In conjunction with a dummy pointer, they can be used as maximum demand indicators. With a switching scheme, they can be used for signal indication of the maximum current. Materials The bi-metallic strip is made of platinum-silver alloy wire. Also in some instruments platinum- iridium alloy is adopted. Bimetallic Instruments 1. Bimetallic spiral
2. Spiral for compensation
3. Strip for leading