Page:Collected Physical Papers.djvu/361

Rh the selenium cell is next exposed to light by an automatic electro-magnetic shutter T. The deflection of the galvanometer is recorded by means of electric sparks on a piece of moving paper. The different operations are carried out in proper sequence by the automatic devices described below.

K₁ completes the battery circuit for about 10 seconds, by which time the record is completed. The successive records for variation of light are taken at intervals of 15 minutes; the periodic closures of the circuit are thus for 10 seconds at intervals of 15 minutes. This short passage of the current is found in practice to cause no polarisation.

The second key K₂ actuates an electromagnetic device by which the trap-door, T, is opened for the definite period of one second; the selenium cell S inside the dark box is thus exposed to light for this length of time. The trap-door is seen in the diagram immediately above the dark box. In reality it is at the upper end of a vertical tube the inside of which is coated with lamp-black to prevent side reflection. The light that falls on the selenium cell is thus from a definite area of the sky. The intensity of light from the sky at different periods of the day causes deflection of the galvanometer which is proportional to that intensity. The maximum deflection of the galvanometer employed is attained in the course of 3 seconds after the exposure.

The third key K₃ is for completion of spark circuit R for record of the maximum galvanometric deflection three seconds after the exposure of the selenium cell. This key actuates a sparking coil R, the vibrating interrupter of which is not shown in the figure. The spark, thus produced, punctures the maximum deflected position