"The trolley-wire must... be suspended with only a very small sag, and to obtain this result without excessive tension in the wire the span must be relatively short, i.e. of the order of 10 ft. to 15 ft. It is obvious that for these short spans the method of construction adopted in tramway practice would be unsuitable, both from the mechanical as well as the electrical standpoint. However, by adopting the catenary system—that is, supporting the trolley-wire from another wire, suspended with considerable sag between supports of moderate span—we are able to obtain a level trolley-wire with a relatively small number of supporting structures. The wire from which the trolley-wire is supported is called the "catenary" or "messenger" wire, and by insulating this wire from the supporting structures there is no necessity for insulated hangers on the trolley-wire."