form was immediately developed into what was called a "focus tube" in which a similar concave focusing cathode was employed, and a platinum plate inclined at forty-five degrees to the axis of the cathode rays was inserted between the two electrodes of the tube to receive the impact of the cathode rays as in No. (4). A plate so placed is called an anticathode. This idea was carried still further to produce the double-focus tube shown in No. (5), which is especially suited to oscillatory discharges from the electrodes, and therefore adapted to use with alternating current apparatus, especially Tesla coils. In this form of tube the anticathode
| 5 | 4 | |
 | ||
| 2 | 1 | 3 |
| Fig. 3.—Typical Forms of Crookes's Tubes. | ||
consists of a wedge-shaped piece of platinum midway between the ends of the tube. If this platinum terminal be connected with the positive pole and both end electrodes with the negative pole, this tube is very efficient with a Ruhmkorff coil giving unidirectional discharges. For any tube there is a critical degree of vacuum as well as electric potential, with which it is most efficient. Tubes can be made suitable for a coil giving a spark of not more than an inch, but they are not very energetic. Since the vacuum rises with continued use of a tube, some forms—e. g., No. (5)—have a small side tube communicating with the main bulb and containing caustic potash or other substance which volatilizes on being heated, so that its vapor will reduce the
