861,582. Measuring instruments. SANGAMO WESTON Ltd. June 14, 1957 [June 18, 1956], No. 18878/57. Class 37. A voltage - measuring instrument having a compressed scale for indicating voltage exceeding a certain level (Fig. 2, not shown) comprises a milliammeter 1 shunted by a series resistor 3 and a silicon rectifier diode in reverse polarity to a unidirectional voltage source 7 to be measured; the combined circuit being connected thereto over a series resistor 5. Up to a critical voltage (e.g. 10 volts), the rectifier has high impedance and the meter operates with resistor 5 as a high sensitivity voltmeter, while when the critical voltage is exceeded the rectifier breaks down to a low resistance in the reverse direction shunting the meter so that its voltage measuring sensitivity is reduced. In a modification (Fig. 3, not shown) the rectifier is shunted by a further resistor effective to increase the breakdown voltage of the diode and shunt the meter while the diode is non- conductive so that the compression of the upper part of the scale is also compressed (Fig. 4, not shown). A theoretical analysis is given. The value of resistor 3 is such that the highest reading of the lower scale range coincides with the lowest reading of the high range.