(a) Field of the Invention
This invention relates to potentiometers and more particularly to improvements in a non-contact magnetic potentiometer using a hall effect device. 
(b) Description of the Prior Art
Various non-contact magnetic potentiometers using hall effect devices are already suggested.  For example, there is known a type wherein a hall effect device is arranged near the periphery of a circular permanent magnet magnetized in the diametral direction so as to obtain a hall electromotive with the variation of the magnetic flux by the rotation of the circular permanent magnet or a type wherein a hall effect device is arranged as opposed to a circular permanent magnet magnetized to have two poles in the rotating direction so as to obtain a hole voltage with the variation of the magnetic flux when the circular permanent magnet is rotated. 
In such magnetic potentiometer, for example, in order to obtain an output voltage of a sine wave form, the permanent magnet disk facing the hall effect device must be magnetized so that its magnetic flux density may be distributed in a sine wave form.  However, such magnetization is difficult.  This is the same also in the case of obtaining an output voltage of such other wave form as a triangular wave form. 
Further, in general, in this kind of magnetic potentiometer, the range of the effective electrical angle is so narrow that, in order to make the electrical angle wise, such contrivance as, for example, of the shape of the yoke is already suggested.  However, it is difficult to make the yoke in any desired shape.  In this kind of magnetic potentiometer, the direction of the magnetic flux acting on the hall effect device is fixed and therefore there is a defect that the variation width of the output voltage is small. 
Further, there are problems that the temperature coefficient of the hall effect device is so large that the drift of the output voltage by the atmospheric temperature will be large and the variation width of the output voltage will become smaller with the time due to the variation of the permanent magnet with the lapse of years.  Therefore, in this kind of magnetic potentiometer, measures must be taken to compensate the variation of the output voltage by the temperature variation and the variation with the lapse of time.  For this purpose, there is already known a method wherein the variation of the output voltage by the temperature variation is canceled by utilizing the temperature characteristics of the permanent magnet and hall effect device a method wherein the temperature is compensated by utilizing a thermistor or the like as a temperature sensor.  However, in such conventional method, as the compensation is made by utilizing the temperature characteristics of each temperature-sensitive element, due to the fluctuation of the temperature characteristics of the hall effect device and temperature-sensitive element, no perfect compensation can be made.  By the way, there is not yet suggested any effective compensating means for the variation with the lapse of time of the permanent magnet whose magnetic force reduces with the lapse of time.