British Kinematography (1951)

156 BRITISH KINEMATOGRAPHY Vol. 18, No. 5 and so resistance only will continue to be measured. I was a little mystified by this change in name. At first I thought it had something to do with the inclusion of the earth leakage tripping coil where reactance forms an important factor, but this is not so because in another part of the Regulations the total impedance of the earth continuity path specifically excludes this earth leakage tripping coil. I do not think it matters whether the term used is resistance or impedance, because in practice resistance only will continue to be measured. Regarding the question how are earth leakage trips to be designed in order to comply with the new requirement. First, you can already buy earth leakage trips — they are designed. It is not necessary to fit those to ordinary circuits. It is only when the earth continuity path is of such resistance that the forthcoming fault would not blow a fuse, and I think it is only necessary to fit these for circuits of over 100 amps., providing you comply with the regulation as to the impedance or the resistance of the earth continuity path. Mr. Garling: As far as the sound equipment is concerned, I cannot see that it matters at all whether it is 1 ohm resistance or 1 ohm impedance. Also as far as the sound equipment is concerned, from the slight amount of study I have been able to give the subject, I think that we are not interested in trip devices because we can claim that in all cases, fuses would blow. Mr. Lambert: Would Mr. Leslie, in his capacity as a Consulting Engineer, be prepared to make a D.C. resistance measurement of the earth leakage path and to certify that the impedance is less than 1 ohm? Mr. Leslie: Yes, I would. The earth continuity path does not include the resistance of the earth as well — it does not include the earth leakage trip. The new Cinematograph Regulations require rewinding apparatus to be earthed. Does the Trust consider that experience prove* this requirement to be necessary? Mr. Pulman : My personal experience proves this to be entirely unnecessary. I think that everybody will agree that a spark from static electricity will not ignite nitrate film. I believe some time ago there was a fire in France, and finally the conclusion was reached that it was caused through discharge of static electricity, but I think again there was some question involved in the processing of the film, and I believe the idea of this requirement is that a nitrate film might be processed with some sort of liquid that might give off a gas which could be ignited by a spark. But I am very sceptical about the whole thing. Mr. Ellis: I have seen static occasionally, and I have not come to any conclusion that it would be dangerous. Mr. Pulman has mentioned a fire in France, but I am very much inclined to think that that static was possibly a cigarette. I do not think it could be proved that static is dangerous or could ever cause a fire. However, if the Regulations say we must earth the equipment, we must do so. Question-Master: A large number of tests have been carried out with the object of igniting film whilst it is being rewound Using new or good quality of film, rewinding at very high speed, and causing considerable friction with blocks, etc., we have been able to generate the most remarkable display of static discharge. We were, however, unable to ignite the film ; nevertheless, we did find that where certain types of cleaning material had been used and the film was re-wound shortly after coming off the projector and, therefore, was warm, inflammable gases were given off which the static discharge could ignite and, indeed, where the film was in its first stages of decomposition, the gas flash was immediately followed by a very substantial explosion. I think, therefore, that it is very wise to earth these winding heads, particularly in view of the fact that there are a large number of kinemas in the country which show old films which have been similarly cleaned and where there have been these mysterious fires. In many cases the projectionist has very wrongly been under suspicion of smok