In this gage, the electrons are generated by a large electric field instead of a hot cathode. The gage consists of two plate cathodes made from zirconium or thorium.
In the space between these cathodes is placed a ring-shaped anode. A magnet provides a magnetic field normal to the plane of the cathode plates, as shown diagrammatically in Fig. 5.36. It is also known as the Penning or Philips gage.
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| Fig 5.36 Cold-cathode vacuum gage. |
A high voltage is applied between the anode and the cathodes, producing electrons that leave the cold cathode surface. These electrons, in the presence of the magnetic field B, travel in spiral paths toward the positively biased ring anode. Only a few electrons are captured by the anode, but the rest make several passes through the ring anode, and in the process ionize the gas.
The positiveions collected by the negatively biased cathodes produce the usual ion current proportional to the gas pressure. The effective range of this gage is 1×10-8 to 2×10-2 torr.
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