struck by the lever 33 as it rotates this causes the pawl 21 to turn clockwise about its pivot 27 and free the disc 19 as well as the rear drum 11 and the rear screen 12 when a battery v is provided and is operating properly it energizes the electromagnet m 2 as soon as a switch s m is turned on this attracts the lever 29 and allows the lever 33 to remain outside the operating range of the pin 20 34 on the disc 20 and avoid hitting the rear screen locking pawl 21 thus as long as the battery v operates properly the shutter performs in the usual manner without operation of the lever 33 when the user forgets to insert the battery v or when the battery v is exhausted so that its voltage is less than a predetermined value closing of the switch s m fails to cause the electromagnet m 2 to attract the lever 29 the spring 37 then holds the lever 29 against the stopping pin 38 as shown in fig3 c and 3d here fig3 c illustrates the condition of the levers when the shutter has just been cocked and the drums 8 8 39 and 11 wound fig3 d illustrates the condition after the disc 20.