Canon AT 1 - Manual (Page 89)

Long Exposures and "B" (Bulb) Setting
When you need shutter speeds slower
than two seconds such as for shooting night
scenes or fireworks, set the shutter speed dial
at “B". Then, the shutter will remain open as
long as the shutter button is pressed. In long
exposures, it becomes essential to mount the
camera on a tripod and use a cable release
preferably with a lock to prevent camera
shake and attain best results.
A cable release with a locking device can
keep the shutter open even though the
operator leaves the cable release unattended.
Unlock the cable release to close the shutter.
Photography using the "B" setting will
accelerate battery consumption since it
requires continuous battery power. When
necessary, the battery should be replaced with
a new one having a full charge.