Wimshurst Static Machine (ca. 1890s) |
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| During the decade following the
discovery of x-rays, the high voltage for x-ray tubes was provided by a
static machine or an induction coil. Static machines were relatively
inexpensive and simple to set up but they fell out of favor for several
reasons: they could not deliver as high a current as induction coils, they
were prone to mechanical failure, and their output was erratic since their
operation could be affected by dust and humidity.
The unit pictured here has two glass disks which rotate in opposite directions - the disks have been coated with shellac to reduce the adsorption of water. There are twenty-two metal "sectors" on the outside edge of each disk. The sectors accumulate a charge when they come in contact with the wire "neutralizing brushes." Every time a sector comes in contact with a brush, its charge changes from positive to negative or vice-versa. The result is that the sectors have a positive charge on one half the disk and a negative charge on the other half. Some of this charge is then removed by the two U-shaped charge collectors. The charge is picked up by the pointed teeth of the collectors and transferred to one of the two glass Leyden jars (capacitors). A Leyden jar is simply a glass jar the bottom of which is lined on the inside and outside with tin foil. The device’s output voltage is estimated by the length of the spark across the spark gap formed by the two metal balls. The smaller of the two balls is connected to the positive terminal. This Wimshurst machine was manufactured by Baird and Tatlock of London before the discovery of x-rays. Later versions, designed specifically for the production of x-rays, were much larger. Some employed more than a dozen disks up to three feet in diameter that were housed ornate wooden and glass cases. |
