ABSTRACT
A contact system is the second essential component of any relay. Usually it consists of several elements (Figure 3.1). Current-conducting elements are made of elastic materials (as a rule, beryllium bronze or phosphor bronze) providing not only current supply to contacting surfaces but also the necessary contact pressure. Contact straps (which are actually contacts) are made of materials of high electric conductivity and resistance to electric erosion. They are riveted, soldered, or welded on by silver solder to currentconducting springs. Contact straps are usually made in the form of rivets or pins (Figure 3.2). Riveted attachment of contact straps is less reliable than welded attachment, because of the considerable increase in transient resistance of a rivet at the point of its joint to the contact spring, caused by heat cycling in course of its exploitation. Such straps, which are often bimetallic (two-layer), consist of a copper base and contact material, an alloy based on silver. Bimetallic contacts of a relay were invented long time ago. Strange as it may seem, it was radio engineering that made the greatest contribution to
the creation of powerful bimetallic contacts resistant to an arc. This field of technology is usually associated in our minds with miniature electronic elements or radio sets. It may seem that radio engineering as a wireless communication facility must have replaced a telegraph with its posts, wire, and sounders and in the long run put an end to wide use of relays in telecommunication systems, but the point is that early radio communication systems differed greatly from those we are familiar with today. Remember, the first radio set was a ‘‘storm indicator,’’ that is, a device registering electric strikes of lightning, but in order to establish radio contact it is necessary to have not only a radio set, but also a radiotransmitter, and what can replace lightning? The answer is obvious: an electric spark, which may be regarded as miniature lightning. Indeed, an electric spark turned out to be a source of electromagnetic radiation in a
wide range of frequencies, including high frequencies, which are quickly spread over
and
long distances. A continuous pulsating arc strikes an electric circuit consisting of a coil, a capacitor, active resistance, an DC power source, and two carbon electrodes (Figure 3.3), and is a power source of radio waves. The first development of the oscillating arc was done by the Danish scientist, Valdemar
Poulsen. Australian born and Stanford educated Cyril Elwell came across Poulsen’s experiments and at once realized their commercial potential. In 1909, with the support of investors, he founded Federal Telegraph in San Francisco, in order to commercially exploit arc technology. One technical drawback of that type of arc transmitter was that it was impossible to
connect a Morse key directly to the circuit of the electric arc in order to receive a controlled (modulated) radio-signal, because the arc ran continuously and it was impossible to switch high powers (tens and hundreds of kilowatts) by means of a manual switch. Here a relay came in useful. Being connected to a DC source, its winding was also in
series with a Morse key. Referring to the schematic, note that the arc, denoted by ‘‘X,’’ is switched by relay contacts, ‘‘K,’’ between the antenna circuit and the dummy load tuned to the same frequency as the antenna circuit (Figure 3.4). In 1913 Elwell developed and demonstrated this technology to the US Navy. By 1921,
80% of all commercial and military transmitters were of the arc variety. The capacity of such transmitters was continuously growing and eventually reached one million watt
(a transmitter installed in Bordeaux, France). Obviously, to produce modulation arcs of such great power, relay transmitters must contain powerful wear-resistant contacts. As can be seen in photos and samples of relays of this type (not the most powerful ones) that have survived until today (Figure 3.5), such contacts consist of two layers of different metals and are quite wide in diameter (3/4 in.). Stationary contacts are supplied with spring-coupler dampers, extinguishing impact energy of a switching contact.
