26 IONIZATION TRANSDUCERS AND APPARATUS EMBODYING SAID IONIZATION TRANSDUCERS AND INCLUDING DIFFERENTIAL PRESSURE METERS, DIFFERENTIAL PRESSURE SENSORS, DIFFERENTIAL VACUUM TUBE VOLTMETERS AND METERS TO MEASURE TEMPERATURE OR PRESSURE OR OTHER VARIABLE TO BE MEASURED; COMMUNICATION APPARATUS EMBODYING LIGHT MODULATORS; COMPARATOR MICROMETERS; ROTOR MICROMETERS; AND DYNAMIC BALANCING MACHINES