Induction heated fixing device and image forming apparatus

1. A fixing device comprising:

a fixing member including a conductive layer and that fixes toner on a recording material while the conductive layer is induction-heated;

a magnetic-field-producing unit including

an exciting member that produces an alternating-current magnetic field intersecting the conductive layer of the fixing member, and

first magnetic-circuit-producing members that each produce a magnetic circuit of the alternating-current magnetic field produced by the exciting member, the first magnetic-circuit-producing members including members provided at a predetermined first interval in a longitudinal direction of the fixing member and members provided at a second interval in the longitudinal direction of the fixing member, the second interval being smaller than the first interval;

a temperature measuring device provided in contact with an inner circumferential surface of the fixing member and that measures the temperature of the fixing member; and

a second magnetic-circuit-producing member having a cut portion in which the temperature measuring device is provided, the second magnetic-circuit-producing member being in contact with the inner circumferential surface of the fixing member and producing a magnetic circuit of the alternating-current magnetic field produced by the magnetic-field-producing unit, the second magnetic-circuit-producing member conducting heat to the fixing member by being induction-heated,

wherein the first magnetic-circuit-producing members at the second interval are provided on both sides of a position corresponding to the cut portion of the second magnetic-circuit-producing member in a direction in which the magnetic circuit is produced, and the first magnetic-circuit-producing members at the first interval are provided at other positions.

2. The fixing device according to claim 1 , wherein the second interval is determined in accordance with reduction in heat capacity of the second magnetic-circuit-producing member that occurs by providing the cut portion in the second magnetic-circuit-producing member.

3. A fixing device comprising:

a fixing member including a conductive layer and that fixes toner on a recording material while the conductive layer is induction-heated;

a magnetic-field-producing unit including

an exciting member that produces an alternating-current magnetic field intersecting the conductive layer of the fixing member, and

first magnetic-circuit-producing members that are provided at a predetermined interval in a longitudinal direction of the fixing member and each produce a magnetic circuit of the alternating-current magnetic field produced by the exciting member;

a temperature measuring device provided in contact with an inner circumferential surface of the fixing member and that measures the temperature of the fixing member; and

a second magnetic-circuit-producing member having a cut portion in which the temperature measuring device is provided, the second magnetic-circuit-producing member being in contact with the inner circumferential surface of the fixing member and producing a magnetic circuit of the alternating-current magnetic field produced by the magnetic-field-producing unit, the second magnetic-circuit-producing member conducting heat to the fixing member by being induction-heated,

wherein the first magnetic-circuit-producing members are provided avoiding an extension of a center line of the cut portion in a direction in which the magnetic circuit is produced.

4. The fixing device according to claim 3 ,

wherein the first magnetic-circuit-producing members include

members provided at a first interval; and

members provided at a second interval smaller than the first interval and on both sides of the extension of the center line.

5. An image forming apparatus comprising:

a toner-image-forming section that forms a toner image;

a transfer section that transfers the toner image to a recording material; and

a fixing section that includes

a fixing member including a conductive layer and that fixes toner on a recording material while the conductive layer is induction-heated,

a magnetic-field-producing unit including

an exciting member that produces an alternating-current magnetic field intersecting the conductive layer of the fixing member, and

first magnetic-circuit-producing members that each produce a magnetic circuit of the alternating-current magnetic field produced by the exciting member, the first magnetic-circuit-producing members including members provided at a predetermined first interval in a longitudinal direction of the fixing member and members provided at a second interval in the longitudinal direction of the fixing member, the second interval being smaller than the first interval,

a temperature measuring device provided in contact with an inner circumferential surface of the fixing member and that measures the temperature of the fixing member, and

a second magnetic-circuit-producing member having a cut portion in which the temperature measuring device is provided, the second magnetic-circuit-producing member being in contact with the inner circumferential surface of the fixing member and producing a magnetic circuit of the alternating-current magnetic field produced by the magnetic-field-producing unit, the second magnetic-circuit-producing member conducting heat to the fixing member by being induction-heated,

wherein the first magnetic-circuit-producing members at the second interval are provided on both sides of a position corresponding to the cut portion of the second magnetic-circuit-producing member in a direction in which the magnetic circuit is produced, and the first magnetic-circuit-producing members at the first interval are provided at other positions.