The gauge kit contains everything you need to measure how much mercury you are drawing while turning i.e. how much vacuum is being generated.
Why would I need a Gauge?
This is very important. It allows you to quickly and easily see if you are losing vacuum. Losing too much vacuum may cause the piece you are turning to come flying off the lathe and cause some serious damage to either you or your shop. It contains a mounting bracket for convenient mounting of the gauge.