202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



MAGNETRON 



As the negatively charged grid cuts off the current of the three- 

 element tube, so an external magnetic field will also do it in the two- 

 element tube, by so influencing or directing the moving electrons 

 that they can not reach the plate. Such a device is called a mag- 

 netron. The particular magnetron used in this experiment, Plate 



2, Figure 2, consisting of a cylindrical plate in a vacuum and a 

 central filament within it, has a coil of wire wound on the outside 

 of the glass. A feeble current sent through the coil from a battery 

 may be made to set up within this coil a magnetic field about equal 

 to that of the earth's magnetic field, so that by moving the whole 

 apparatus about in space (thereby at one time adding to the earth's 

 field, at others opposing it) the magnitude and direction of the 

 earth's magnetic field can be disclosed by a meter which indicates the 

 resultant current. When the magnetron points toward the north 

 pole, the meter shows no current, while in other positions currents 

 are measured. 



AXIOTRON 



Another useful vacuum tube of this type is one in which the mag- 

 netic field of the filament current itself becomes great enough during 

 each current cycle to deflect the electrons so that they will not reach 

 the plate. By this tube, " the axiotron," the frequency of an alter- 

 nating current may be doubled or direct current be changed into 

 alternating. 



PHOTO-ELECTRIC CELL 



Another vacuum tube is the photo-electric cell. As shown in Plate 



3, Figure 1, one of these may be connected with a relay to a lamp 

 so that when light shines upon the cell the burning electric lamp 

 is extinguished, and relighted on cutting off the light from the 

 photo-electric cell. In other words, it turns on the lamp when it is 

 dark and turns it off when it is light. This depends on the fact that 

 some metals, like potassium, emit electrons when light falls upon 

 them. These electrons in vacuum constitute a current when they 

 are made to move by the electrical impulse. To repeat; applying 

 potential to a vacuum tube having a potassium electrode and an- 

 other electrode for leading off the current causes current to flow 

 in the form of negative electrons from the illuminated metal, and 

 this current actuates the electrical switch which turns off the lighting 

 current of the burning lamp. 



THE NERVE 



This leads to the next, an ambitious physiological experiment 

 As shown in Plate 3, Figure 2, a photo-electric cell may be made to 



