The precision wire wound type potentiometer is widely used as an electromechanical transducer for self balancing type instrument, electronic analog computer, etc. because of its excellent features such as high accuracy, simple structure and easy handling.
Excellent it is, it has some big difficulties in practical uae, such as the wear of the wire and contact, increase of contact noise, etc., which often have beeh discussed up to this time. Still, it has never been paid attention at all that the thermal EMF in the potentiometer circuit affects some characteristics of the self balancing type instrument.
In this paper, the above fact is discussed both experimentally and theoretically. The thermal EMF of the potentiometer circuit in the self balancing type instrument can be classified into the following two types:
1. Static thermal EMF
2. Dynamic thermal EMF
(a) Mean temperature rise of contact.
(b) Local and short-time thermal flash at contact.
The static thermal EMF is generated by the temperature gradient which exists in each component made of different kinds of metals. Its malfunction appears as a long time drift of indication of the instru ment. The dynamic thermal EMF is generated by the temperature rise of the contact caused by sliding friction, (a) of the above classification affects dynamic characteristics of the self balancing type instrument and (b) is considered to have close relations with the wear of the wire and contact and with the increase of the contact noise of the potentiometer.
The authors made it possible to explain the mechanisms of the dynamic EMF and its influences to the self balancing type instrument. Through this analysis, the transfer-function of the potentiometer circuit,
GH, is represented as follows:
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During its derivation, the temperature rise of the contact is assumed to follow Newton's low ol cool ing. The result of the calculation by using the above transfer-function well proves the experimental fact that the dynamic response of the itstrument is different by sliding direction.
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