The Monitor 4 radiation detector is a compact general purpose survey meter capable of detecting alpha, beta, gamma, and X-rays over 3 selectable ranges. It is in a small handheld black plastic case. The front of the machine has two switches and a meter in the readout screen. The scale in the readout screen has 0-5µ Sv/hr (sieverts per hour) in the upper scale and 0-0.5 mR/hr (rem) in the lower scale. The range switch consists of three settings: x1, x10, and x100. The whole unit has a leather carry case.
The monitor senses ionizing radiation by means of a GM (Geiger Mueller) tube with a thin mica window. The tube is closed inside the instrument. When a ray or particle of ionizing radiation enters or passes through the tube, it is sensed electronically and displayed by a red light count. When the switch is in the AUDIO position, the instrument will also beep with each radiation event, about 5 to 25 counts at random intervals (depending on your location and altitude) can be expected every minute from naturally occurring background radiation.