"actuates a detonator inside the flask"
Extract from the Ilford Manual of Photography showing a pneumatic camera shutter
An India-rubber ball is a hollow ball connected to a tube which pumps air when squeezed. The detonator which the Professor carries about with him to set off his personal bomb is a chemical detonator, which works by mingling explosive substances which react on contact with one another. He likens it to the pneumatic camera shutter, which came in in the last quarter of the nineteenth century. This worked by pressure from a rubber ball triggering a piston, which tripped the shutter, taking the photograph.