inside of the premises or in a protecting case, if outside, on account of the necessity of shielding both of these devices from the weather.  Speaking generally, the wider practice is to put all the elements of the subscriber’s station protector inside of the house.  It is nearer to the ideal arrangement of conditions if the protector be placed immediately at the point of entrance of the outside wires into the building.

[Illustration:  Fig. 230.  Western Electric Station Arrester]

[Illustration:  Fig. 231.  Cook Arrester for Magneto Stations]

Ribbon Fuses.  A point of interest with relation to tubular fuses is that in some of the best types of such fuses, the resistance material is not in the form of a round wire but in the form of a flat ribbon.  This arrangement disposes the necessary amount of fusible metal in a form to give the greatest amount of surface, while a round wire offers the least surface for a given weight of metal—­a circle encloses its area with less periphery than any other figure.  The reason for giving the fuse the largest possible surface area is to decrease the likelihood of the fuse being ruptured by lightning.  The fact that such fuses do withstand lightning discharges much more thoroughly than round fuses of the same rating is an interesting proof of the oscillating nature of lightning discharges, for the density of the current of those discharges is greater on and near the surface of the conductor than within the metal and, therefore, flattening the fuse increases its carrying capacity for high-frequency currents, without appreciably changing its carrying capacity for direct currents.  The reason its capacity for direct currents is increased at all by flattening it, is that the surface for the radiation of heat is increased.  However, when enclosed in a tube, radiation of heat is limited, so that for direct currents the carrying capacity of fuses varies closely with the area of cross-section.

City-Exchange Requirements.  The foregoing has set down the requirements of good practice in an average city-exchange system.  Nothing short of the general arrangement shown in Fig. 225 meets the usual assortment of hazards of such an exchange.  It is good modern practice to distribute lines by means of cables, supplemented in part by short insulated drop wires twisted in pairs.  Absence of bare wires reduces electrical hazards enormously.  Nevertheless, hazards remain.

Though no less than the spirit of this plan of protection should be followed, additional hazards may exist, which may require additional elements of protection.  At the end of a cable, either aerial or underground, long open wires may extend into the open country as rural or long-distance circuits.  If these be longer than a mile or two, in most regions they will be subjected to lightning discharges.  These may be subjected to high-potential contacts as well.