Photo's description:

Circa 1947-1950. Close-up of the apparatus used to test the hlding power of energized iron particle-in-oil mixture. MBS staff member Patricia Hickok is shown holding the 2-inch square plate which is immersed in a magnetic fluid filling the chamber between the pole pieces of the electromagnet assembly on the bench in front of her. When 10 watts of power are supplied to the coil, the plate will sustain a pull of 200 pounds without moving. The "chains" that develop when the magnetic coil is energized are shown in NBS photo 1500. Jacob Rabinow invented the Magnetic Particle Clutch in 1947 during his work on ordnance at the National Bureau of Standards. The particle clutch used a mixture of light lubricating oil and powdered carbonyl iron inside a chamber connecting two free spinning plates. When an electromagnet attached to the chamber was energized, the iron particles would magnetize and attract each other, producing an almost solid mass, which locked the plates together. Controlling the magnetic flux would control the amount of torque between the plates. Because Rabinow invented the Magnetic Particle Clutch as part of his work at the National Bureau of Standards, the United States Government owned the U.S. patent. However, in partnership with his brother Joseph, Rabinow did file for patents in 22 foreign countries. Due of the simple design, precise torque control, smooth operation, and long life, the device found wide application. It was used in Renault and Subaru automobiles in Europe and Japan, in airplane controls, and in the disc file of the IBM RAMAC computer. Reference: Inventing for Fun and Profit. Jacob Rabinow. San Francisco Press, San Francisco, CA. (1990). pp. 49-55.