Device for maintaining constant the temperature of a coil fed by an A.C. current source

In a device for maintaining constant the temperature of a coil fed by an electrical alternating current source, said coil comprising two windings (A and B), each of said windings being formed by winding simultaneously two insulated wires (a.sub.1, b.sub.1 and a.sub.2, b.sub.2 respectively) having the same size, said two wires of each winding extending side by side and having each a first and a second end, said two windings having the same number of single turns wound in the same direction, means connecting the second end of the first wire (a.sub.1) of the first winding (A) to the first end of the first wire (a.sub.2) of the second winding (B), means connecting the second end of the second wire (b.sub.1) of the first winding to the second end of the second wire (b.sub.2) of the second winding, and means connecting the first end of the first wire (a.sub.1) of the first winding and the second end of the first wire (a.sub.2) of the second winding across said electrical alternating current source, the improvement comprising:

• a. a Wheatstone bridge comprising four arms, the first end of the second wire (b.sub.1) of the first winding (A), and the first end of the second wire (b.sub.2) of the second winding (B) being connected into the bridge to form one leg thereof, the other three legs consisting of resistors, such that said second wires are traversed by an electric current in opposite directions and that the resistances of said second wires (b.sub.1, b.sub.2) are balanced by said resistors,
• b. a differential amplifier having two inputs, said inputs being connected to the end terminals of a first crossarm of said bridge, and
• c. a voltage-controlled direct source having two terminals and a control input, said two terminals being connected to the end terminals of the second crossarm of said bridge and said control input being connected to the output of said differential amplifier.