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As we all know ’Ionising’ radiation (Alpha, Beta, Gamma, X-Rays
and so on) can be tricky stuff. Those who's business it is to handle or come in
contact with radioactive materials and X-Rays on a regular basis are generally
very well protected and have to follow ultra strict safety protocols designed
to keep their exposure to well within defined safety limits. But things can and
do occasionally go wrong; accidents and faults happen and that’s when those at
risk need to be rapidly informed that they should step away, quickly!
From the late 70s until comparatively recently an Mk1 Mini
Instruments MiniAlarm 7-10 would have been a welcome and reassuring sight
almost anywhere radioactive materials are processed, stored or handled. It was
a development of the very successful range of 
Green is good, and as long as it stays lit all is presumably well.
And it’s not just the environment that the 7-10 is monitoring. It checks it’s
own status as well with a fail-safe facility that lights up the red panel and
sounds a loud buzzer if the instrument’s probe, connecting cable or probe
supply fails. Also prominent on the front panel is an analogue meter. The
readout is in ‘counts’ per second (CPS), from 0 to 2,000 CPS. The meter is one
of Mini Instrument’s trademark features, still being used on its current range
of workhorse Geiger Counters. The clever part is the logarithmic meter scale,
which does away with a range switch and shows, at a glance, normal background
and relatively benign levels (0 – 100 cps) on the first two thirds of the
scale, and more concerning to scary levels (100 – 2000cps), on the last third
of the scale. 
Two of the front panel controls (Set Trip switch and an adjustable
Trip Level control) are concerned with setting the point at which the alarm
goes off and the red indicator lights up. There’s also a push-button for
testing the alarm functions and a switch that latches the alarm, even if the
levels fall back into the safe zone. It has two front panel sockets; on the
left is a PET connector for the cable that links the instrument to its probe
(in this case a Gamma/X-Ray sensitive Type MC-70). The DIN socket on the right
is a switched relay output for an external or remote alarm. It is mains powered
and there is no On/Off switch. This is another confidence feature in that it
should always be switched on showing a green (or red light) and it helps to
reduce the chance of it being switched off by accident. 
The MiniAlarm 7-10 is housed in a tough metal case and can be used
on the bench though more often than not it would be wall-mounted or sat on a
shelf. The sensor probe can be attached to the unit or, using a long cable,
placed close to key locations where people and radiation are not supposed to
mix. This one is in tip-top condition and good working order and was a swapsie,
with a fellow Geiger nut.