The "elbow" goes between the poles of a very large permanent magnet. The gas or vapor to be analyzed is introduced through the pipe. A hard vacuum is then pulled on the unit and a high voltage is used to ionize the molecules, which are accelerated to a particular speed (controlled by the voltage and the current applied to the plates), and sent around the bend in a strong magnetic field. The speed/mass of molecule and the magnetic field strength combine such that only molecules of a particular mass make it around the bend; the others crash into the walls. The ones that make it hit the multiplier give a current output - so the mass selection is swept through the desired range and current measured, and a quantitative display of molecular masses in the sample can be computed.

Thanks to Danial Stocks, Jim RabidWolf Hickinbotham, Nick Huwar and Ludwell Sibley who helped to identify this device.