The tip of a thermal probe is placed at the focus of the parabolic reflector on the right. An electric heating filament is, in stages, moved closer to the focus of the parabolic reflector on the left. The output of the thermal probe registers little rise in temperature until the heating filament is placed at the focus of the opposite reflector, whereupon the temperature rises to 130 degrees celsius in less than a minute. If a match, tied to a string, is lowered so that its head is placed in the focus of the receiving parabola, the match will ignite after about 20 seconds.