JPS6239938B

PURPOSE:To decrease the number of parts and to elevate an operating property, by setting a current of a plurality of correcting coils by use of one variable resistor, and also making it have 2 functions of rough adjustment and fine adjustment. CONSTITUTION:Voltage consisting of +V and -V applied to terminals 1, 2 is split by a variable resistor 20 and is inputted to an operating circuit 41 through an A/D converter 21. A variation quantity detecting circuit 22 monitors a variation of increase or decrease of the A/D converter 21, and varies contents of registers 26-29 through an adding and subtracting circuit 23 and a switching circuit 24 in case when there is a variation. In this case, in case when the quantity of variation of increase and decrease of the A/D conveter 21 is large, a large coefficient is multiplied, and in case when the quantity of variation is small, a small coefficient is multiplied. On the other hand, to each correcting coils 13-16, each D/A converter 30-33 is connected through resistances 8-11, and a current flows in accordance with the quantity set to the registers 26-29. In this way, the number of variable resistors can be decreased, also the magnetic field correction control can be executed by a computer, and the operating property is elevated.