The invention is directed to a dose control apparatus. The apparatus has two armatures pressed against a valve seat by at least one spring. A coil induces a magnetic field that motivates the armatures against the force of the spring, thereby opening the valve. The armatures may move along a common axis in opposite directions. The apparatus may also include a core located between the armatures and a casing about the coil. The core and casing act to guide the magnetic field, reducing the power requirements for creating the field. Current may be periodically reversed in the coil to provide a degaussing field. In addition, a signal may be produced by the coil in the presence an externally applied magnetic field such as an MRI. An opposing magnetic field may be produced by the coil or the current provided to the coil may be adjusted.