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1938 - enlargement of electricity's finger-print from the results of cable destruction tests


CD 2478074 E&MP14.001

Electric Cables

June 29, 1938

Enlargement of a small area of same sheet of paper (indicated by circular area in each photograph), showing how high intensity electrons or ions pounded away at the paper, branching out over its surface until they eventually found a weak spot and burned through, only to begin operations again on the next layer.

additional text found...


Microphysical and microchemical methods are employed in "crime" detection by Dr. H.H. Race of the General Electric Research Laboratory at Schenectady. The culprits are electrons or ions and the crime is the destruction of electric cables undergoing severe laboratory tests.

For several years Dr. Race has been working in the Schenectady Laboratory with miniature samples of high-voltage oil-impregnated paper-insulated cables. Under high-voltage stresses, electronic or ionic bombardment of the paper causes it to carbonize and burn through, layer by layer, leaving behind the culprit's finger-prints.

An examination of the paper will show, for instance, how the bombardment pounded away at the paper, branching out over its surface until the ions or electrons eventually found a weak spot and burned through, only to begin operations again on the next layer of paper.

As these agents of destruction progress through the layers of paper they tend to spread out, damaging larger areas until complete electrical failure occurs.

With the damaging evidence available, the identity of the culprits is unquestionable and the laboratory investigator can thereby assist the engineer in improving cable design.

Previous to the type of experiments with miniature samples now being conducted in the Schenectady laboratory only full-sized lengths of cable could be tested. By the new method, life test data have already been obtained on many hundreds of different combinations of oils, papers and designs; and analyses of these data are giving a good understanding of the direction cable research should take in the endeavor to obtain improved products.

Large electrical cables, operating at thousands of volts, form the main arteries by which millions of kilowatt-hours of electric energy are conducted from power to distributing stations in the big cities of this country. To insure an increasingly better quality for such cables so they will operate faultlessly for decades is the goal of research engineers.

The combination of pure mineral oil and wood-pulp paper from which water, air, and other contaminants have been very carefully removed makes the best flexible high-voltage electric insulation yet discovered. In building a cable, many layers of paper are wrapped on the copper core in strips only a few thousandths of an inch thick. This paper is then carefully dried and impregnated with oil.

In improving such cables, much importance is attached to the researches in which miniature specimens made in the laboratory under the most carefully controlled conditions are subjected to much more severe conditions than will ever be expected of a cable in service. Such experiments are much less expensive than the large-scale tests; thus, the number of combinations of materials and conditions that can be investigated is correspondingly much greater.

It is from hundred of such experiments that scientists seek to learn how the best possible combination of materials can be assembled to make cables which shall supply electrical life blood to all industries and inhabitants of our metropolitan districts.

One instrument used in the investigation illustrates the minuteness which such miniature tests demand. It was developed so that the electrical condition of each layer of a cable sample can be studied after it has been subjected to severe electrical tests.

The electrical quality of one drop of oil can be measured with this device and, as a cable specimen is dissected for investigation, a few square inches from one layer of paper are enough to yield this quantity of oil. By such means, incipient deterioration, which, in long service, might lead to failure, can be detected, and the causes can be diagnosed and corrected.

Thus can the scientists in the laboratory assist the engineer in the factory in making continued improvements in one of the types of equipment necessary to make available, wherever wanted, man's cheapest servant: electricity.

Original Caption by Science Service
General Electric

National Museum of American History


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