Polonium ionization potentials

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Another method for measuring Volta potentials is to ionize the air between the plates, and adjust the potential applied to them until no current passes across the air gap. This method appears to have been used first by Righi2 (with ultra-violet rays as a source of ionization), later by Perrin and many later workers, using radium salts 8 Greinachcr,4 and Anderson and Morrison,6 pointed out that errors frequently arose if sources capable of ionizing the air in other parts of the apparatus than directly between the plates and it is well to use either a carefully shielded source of j3 or y rays or a radioactive source such as polonium, which gives off only a rays which have a range of a few centimetres only. This method is that used for the determination of the surface potentials of insoluble films as described in Chapter II. 

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