There are several methods for the measurement of the thickness of electro-plated film on metals; e.g., microscopy, dropping method (jet method), magnetic method, electro-chemical method, X-ray method, high frequency eddy current method (skin effect), etc. The values measured by the above methods are not decisive, since each method has both merits and demerits.
Direct observation of the film with light-wave interference fringes was attempted to determine the distance between the both surfaces of plated and non-plated metal with wave-length and the number of fringes, using the interference microscope made by Olympus Optical Co., Ltd. partly improved to be applied for the measurement of electro-plating thickness.
As a result of series of tests, it was found that, by this method, the thickness of electroplated film could be acculately determined from 0.05 to 40μ.

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Concentrating rinse water in plating solution by electrolysis with ion-exchange membrane was studied using various kinds of cyanide plating solution watered 10 times as dilute as the original.
Result:-
1. Concentration-ratio of each constituent in the diluted zinc cyanide solution is higher in order of OH^-, CN^- and Zn ion.
2. Concentration-ratio of each ion in the diluted silver cyanide solution is higher in order of CN^-, Ag(CN)₂^- and CO₃^--.
3. In the concentration of diluted copper cyanide solution, the concentration-ratio of Cu(CN)₃^-- is greater than that of free CN^-.
4. Current efficiency of ions entering concentration chamber through ion-exchange membrane is 60.3-73.3% when the membrane CMG 10 and AMG 10 is used, and 33.5-46.6% when the membrane CMG 20 or AMT 20 is used.
5. As electrolysis proceeds, the movement of water through the membrane gradually outdoes that of irons, so that the total gram-molar concentration-ratio arrives at the maximum value showing no further increase. The maximum value is 5.15 for diluted zinc cyanide, 12.26 for diluted silver cyanide and 10.46 for diluted copper cyanide solution respectively.

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