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Radio Broadcast
by the material itself so much as by the film which accumulates on it. The film is composed largely of moisture condensed from the humid atmosphere. In general, hard substances have more surface leakage than soft substances. At high humidities, there may be a change of ten per cent, on account of surface leakage, and this deficiency lasts for several hours and sometimes as much as a month after the humidity is lowered.
Since good oil is one of the best insulating materials, it can be used to avantage in many places. When moisture meets an oily surface it tends to collect in drops which otherwise might spread over the surface and cause a filmy leakage path.
Air is an insulator, but is not so good as the ordinary solid insulators. Where bare conductors come close together they should be kept apart by some solid insulating substance.
In the case of condenser action, insulators exhibit another property quite out of proportion to their insulating power. It is called the dielectric constant and must be considered when building a condenser. Air is taken as the unit of measurement in the accompanying table and the constants of other materials are figured in proportion to that of air:
TABLE* OF COMPARATIVE RESISTANCES OF VARIOUS INSULATORS TO HIGHFREQUENCY CURRENTS
The figures given must be multiplied by i ,000,000,000,000,000 to find the ohms resistance of a cubic centimeter.
Amberite . . 50
Bakelite .004-20
Beeswax 2
Celluloid 00002
Condensite 00004
Dielectrite "... .005
Electrbse 1-20
Fibre, hard 00002
Fibre, red 000005
Glass OOI-.O5
Halowax 02-20
Hard rubber 1000
Marble 000005
Mica 2.-20O
Paraffin, special over 5,000
Paraffin, ordinary lo.-ioo
Porcelain 3
*Prepared by the Bureau of Standards
Quartz over 5,000
Redmonite 2
Rosin 50.
Sealing Wax 8.
Shellac 10.
Slate ooooooi
Sulphur 100.
Wood, mahogany 04
Maple 00003
Poplar 0005
TABLE SHOWING CHANGE OF RESISTANCE WITH TEMPERATURE
These figures show the ratio of the resistance at 20 degrees Centigrade to that at 30 degrees, based on experiments of the Bureau of Standards.
Sealing Wax 9
Mica i to 2.
Shellac 1.5
Celluloid 1.8
Parawax 2
Bakelite 2.4 to 3.6
German glass 2.5
Red fibre 2.6
Hard fibre 3.2
Plate glass 3.2
Paraffined wood 3.6 .
Beeswax 16.
TABLE OF DIELECTRIC CONSTANTS AS USED BY THE SIGNAL CORPS
The capacity of a condenser using various
substances as the dielectric is given in ratio to the capacity of the same condenser using air as the dielectric.
Air . . . . i.
Glass 4 to 10
Mica 4 to 8
Hard Rubber 2 to 4
Paraffin 2 to 3
Paper, dry 1.5 103
Paper, treated as used in cables . . . . 2.5 to 4
Porcelain, unglazed 5 to 7
Sulphur 3 to 4.2
Marble 9 to 12
Shellac 3 to 3.7
Beeswax 3.2
Silk 4.6
Celluloid 7 to 10
Wood, maple, dry 3 to 4.5
Wood, oak, dry 3 to 6
Molded insulating material, shellac base . . 4 to 7 Molded insulating material, phenol base
(bakelite) 5 to 7.5
Vulcanized. fibre 5 to 8
Castor oil 4.7
Transformer oil 2.5
Cotton seed oil . . . 3.1