1. Indian Naval Physical Laboratory, Cochin.
2. National Physical Laboratory of India, New Delhi. Sept. 23.

Abstract

SHELLAC is produced in India after refinement from natural seedlac which contains more than 88 per cent of lac and 4.5 per cent of wax, the remainder being made up of colouring matter, gluten and moisture. Various grades of commercial shellac contain varying percentages of soluble impurities, wax, rosin and moisture. For microwave work a very high degree of purity is needed in dielectric samples. The three grades of shellac, 'Blonde', 'Crown' and 'Granta', manufactured by Angelo Bros., are free from rosin and orpiment and contain only 0.5–0.1 per cent of the insoluble impurities. They are at present the only samples of the highest degree of purity available and have therefore been used for dielectric measurements in the 8-mm. band. Measurements have also been taken in two other electrical grades used for micanite. The dielectric loss in shellac, grade 'Blonde', is found to be the lowest in the 8-mm. band, and for this grade measurements of loss have also been carried out over a very wide band of radio frequencies.