St. John, Thomas M. (Thomas Matthew), b. 1865 · New York, T. M. St. John · leaf 145
inary pressures.The nature of the spark is greatly changed when the pres-sure of the air decreases. Fig. 216 shows an air-tight glass tube so arranged that theair can be slowly removedwith an air-pump. Theupper rod shown can beraised or lowered to in-crease the distance betweenit and the lower rod,… … X-RAYS—HUMAN BONES PHOTOGRAPHED. 143 moved, a change takes place. The spark is not so in-tense and has no definite path, there being a generalglow throughout the tube. As the air pressure becomesstill less, the glow becomes brighter, until the entire tubeis full of purple light that is able to pass…
Electricity
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