Abstract: A telephone communication method provides simultaneous duplex channels between respective pairs of stations over a single wire pair highway by frequency division multiplexing. Each duplex channel has a predetermined pair of carrier frequencies, but only carrier frequency pairs of channels in use for communication are present in the highway at any time. On initiating a call, a station seeks for a free channel and reserves it by generating the "go" carrier of the free channel modulated with the call sign of the desired station, and also generating an alert signal. Quiescent stations respond to the alert by seeking a "go" carrier with call sign data and, on identifying their own call sign, generating the corresponding return carrier to set up the channel.

Abstract: A respirator is sealed at its sides to the face of a wearer by seals or air flow restricting members comprising a multiplicity of bristles. The length of the bristles is chosen so that they are deflected to lie at their free ends against the wearer's face. The length resilience and packing density of the bristles is selected to provide a required resistance to air flow. The bristles are of horse hair, camel hair, nylon, polypropylene or other noninjurious material. Preferred bristle diameters are 0.03 to 0.25 mm and lengths 10 to 30 mm.