Potentiometer
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a potentiometer, and more particularly to a novel potentiometer which employs two kinds of resistors arranged in parallel to each other. 
     2. Description of the Prior Art 
     The conventional potentiometer has a single resistor which has input terminals at both ends thereof, and a movable contact slidable along the resistor. When the potentiometer is repeatedly used, the contact resistance between the movable contact and the resistor changes. Particularly in the case where the potentiometer is repeatedly used a great number of times as in a photographic camera wherein potentiometers are used for setting exposure information such as aperture size, shutter speed and film sensitivity in an automatic exposure control circuit, the contact resistance rapidly changes and accordingly the life of the potentiometers is short. 
     SUMMARY OF THE INVENTION 
     In view of the short life of the conventional potentiometers, the primary object of the present invention is to provide a potentiometer which has a high durability and a long life. 
     Another object of the present invention is to provide a potentiometer in which the surface of the resistor on which the movable contact slides has a high durability and accordingly the contact resistance does not readily change. 
     Still another object of the present invention is to provide a potentiometer in which the change in the contact resistance is negligible. 
     The potentiometer in accordance with the present invention is characterized in that a first resistor of low resistance and a second resistor of high resistance are arranged in side by side relation and in contact with each other. The first resistor of low resistance has input terminals at both ends thereof. A movable contact is slidable along the first and second resistors in slidable contact with the second resistor of high resistance. 
     Since the movable contact slides on the second resistor of high resistance, the amount of electric current flowing through the contact point is small and accordingly the surface of the second resistor and the contact point of the movable contact are not greatly damaged and thus have a long life. 
     The above objects, other features and advantages of this invention will be made more apparent from the detailed description of the preferred embodiments thereof taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of an embodiment of the potentiometer in accordance with the present invention, 
     FIG. 2 is a plan view of the potentiometer as shown in FIG. 1, 
     FIG. 3 is a plan view of another embodiment of the potentiometer in accordance with the present invention, 
     FIG. 4 is a circuit view of an equivalent circuit of the potentiometer in accordance with the present invention, and 
     FIG. 5 is a circuit view of an example of an electric circuit employing the potentiometer in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an embodiment of the potentiometer in accordance with the present invention which employs straight resistors arranged in parallel in side by side relation. The potentiometer shown in FIG. 1 is composed of a first resistor 1 having comparatively low resistance, a second resistor 2 having comparatively high resistance arranged in parallel to and in side by side relation with the first resistor 1, and a movable contact 3 slidable along the two resistors in slidable contact with the second resistor of high resistance. The first resistor 1 of low resistance is provided at both ends thereof with input terminals 11 and 12. 
     The first and second resistors 1 and 2 are disposed on an insulating base plate 10. The resistors 1 and 2 are made, for example, of burned paste or a vacuum evaporated material applied to the surface of the insulating base plate 10 in side by side relation as shown in FIG. 1. A pair of conductive parts 13 and 14 made of conductive material such as silver paste are disposed on the base plate 10 in contact with the both ends 1a and 1b of the first resistor 1. Said input terminals 11 and 12 are fixed to the conductive parts 13 and 14, respectively. 
     The movable contact 3 has a contact point 3a which is in slidable contact with the surface of said second resistor 2 of high resistance and is fixed to a slidable member 4 which is in screw engagement with and movable along a screw rod 5 extending in parallel to said resistors 1 and 2 as the screw rod 5 revolves. The slidable member 4 is further slidable engaged with a guide rail 6 extending in parallel to the screw rod 5. The slidable member 4 is made of an insulating material and is provided with an output terminal 15 which is electrically connected to the movable contact 3. 
     FIG. 3 shows another embodiment of the potentiometer in accordance with the present invention. In this embodiment, all the elements equivalent to those shown in FIG. 1 are designated with the same reference numerals. In this second embodiment, a first resistor 1 and a second resistor 2 are arcuate in form and a movable contact 3 is swingable about an axis. Other elements and structures are all the same as those of the first embodiment as shown in FIGS. 1 and 2. 
     The electric characteristics of the above described potentiometers shown in FIGS. 1 to 3 can be represented by an equivalent circuit as shown in FIG. 4, in which the elements equivalent to those shown in FIGS. 1 to 3 are all designated with like reference numerals. In FIG. 4, a resistor R1 is connected between a pair of input terminals 11 and 12. A movable contact 3 with at an end thereof an output terminal 15 is connected with a second resistor R2 by way of a contact point A. A resistor R0 shown between the output terminal 15 and the contact point A represents the contact resistance between the contact point 3a of the movable contact 3 and the surface of the second resistor 2. There is a second resistor R2 between the contact point A and a point B on the first resistor R1. The point B represents the point on the side of the first resistor 1 which is in contact with the second resistor 2 and which is at the position closest to said contact point 3a of the movable contact. These points A and B are shown in FIG. 2. Therefore, the point B moves along the edge of the first resistor 1 as the movable contact 3 moves along the second resistor 2. The second resistor R2 shown in FIG. 4, therefore, represents the resistance of a part of the second resistor 2 lying between the contact point 3a (or A) and the point B. 
     In the equivalent circuit as shown in FIG. 4, the second resistor R2 does not change since the distance between the contact point 3a of the movable contact 3 and the edge of the first resistor 1 does not change as the movable contact 3 moves along the second resistor 2. The contact resistance R0 between the contact point 3a of the movable contact 3 and the surface of the second resistor 2 is somewhat changed as the movable contact 3 moves along the surface of the second resistor 2. However, since the contact resistance R0 is much smaller than the second resistance R2, the change in the contact resistance R0 is negligible. Further, as explained hereinafter, the resistance of the contact resistance R0 and the second resistor R2 is negligible with respect to the divided voltage effected by the division of the first resistor R1 by use of an amplifier of high input impedance connected with the output terminal 15 of the potentiometer. 
     In the practical circuitry, the first resistor R1 has several hundred to several thousand ohms, the second resistor R2 has several hundred kiloohms to several megaohms and the contact resistance R0 is several milliohms to several ohms. Therefore, the contact resistance R0 is negligible with respect to the resistance of the second resistor R2. Further, when the output of the potentiometer is amplified through an amplifier with a high input impedance of 10 9  to 10 10  ohms, the resistance of the second resistor R2 and the contact resistance R0 are negligible. 
     An example of an electric circuit which employs the potentiometer in accordance with the present invention will hereinbelow be described with reference to FIG. 5. Two input terminals 11 and 12 of the first resistor R1 are connected to the positive and negative terminals of an electric power source 7. The output terminal 15 of the potentiometer is connected to an input 8a of an amplifier 8 with a high input impedance. The output voltage of the potentiometer at the output terminal 15 therefore is taken at the output 8b of the amplifier 8. Since the input impedance of the amplifier 8 is much higher than the resistance of the second resistor R2 and the contact resistance R0, the resistance is negligible. 
     In the potentiometer in accordance with the present invention, the movable contact 3 slides along the second resistor of high resistance. Therefore, there flows a small amount of electric current through the contact point between the movable contact and the second resistor. Accordingly, the surface of the resistor and the contact is not damaged by sparks and has high durability. 
     The amplifier 8 which has a high input impedance can easily be obtained, for instance, by attaching a source follower circuit of an FET to the input of an amplifier. Further, there have been know various transistor amplifiers with a high input impedance.