A digital amplitude modulation apparatus is provided with a measuring section, a fast surge protection section, a slow vector protection section, and a controller. An upright arm at one end of the base extends adjacent the web of the rail and carries a threaded adjuster which engages the transducer and urges it into contact with the rail. A valve associated with the fuel supply line. The first wave modulates the core refractive index of the resonator and thereby shifts the resonant conditions of the resonator. Specific embodiments provide message flows among a mobile station, gateway support node, router, and authentication server that support providing prepaid services in a packet-switched network for mobile communication. The adapter part additionally contains measures for accommodating a ground contact.