Abstract: In telephone installations it is often necessary to know, without actually physically going off-hook, the status of the line. A solution to this problem is presented by the design of a low drain line status indicator circuit. Advantage is taken of the fact that the idle line voltage available from the switching machine is significantly greater than the busy line voltage. Accordingly, when the line is idle, a pulsating voltage is developed which is used to flash a light source at the station. When the line is busy or on hold, which situation can occur from a telephone station set connected to a remote extension, the line voltage is insufficient to enable the pulsating voltage circuit and, thus, the light source remains dark. In situations where one telephone station set serves two lines, the light sources associated with each line at each station location are arranged to flash in synchronism.

Abstract: The arrangement of the individual air dampers with respect to the heating coils in a vertical tube face and bypass air treatment unit such that the dampers each move laterally across the coils results in a substantial reduction in the number of moving parts required to control air flow around the coils. Selective proportioning of the air flow is accomplished by splitting each damper into two sections and permanently fixing one section with respect to the coil set while allowing the other section to slide across the face of the coil set in mating relationship with the fixed section.