Line construction from the continuous-current transmission is of the most simple character apart from the necessity of high insulation. Only two wires are necessary and they may be of any desired cross-section, strung on a single pole line and need not be transposed. On these wires the maximum voltage for which insulation must be provided is the nominal voltage of the system. It is possible under these conditions to build a single transmission line with two conductors of such size and strength and at such a distance apart that a high degree of reliability is attained against breaks in the wires or arcing between them. In a transmission of power by two- or three-phase alternating current at least three wires are necessary and six or more are often employed. If six or more wires carrying current at the high voltages required by long transmissions are mounted on a single line of poles, it is not practicable to obtain such distances between the wires as are desirable. The repair of one set of wires while the other set is in operation is a dangerous task, and an arc originating between one set of the wires is apt to be communicated to another set. For these reasons two pole lines are frequently provided for a transmission with alternating current, and three or more wires are then erected on each line. Compared with a continuous-current transmission, one with alternating current often requires more poles and is quite certain to require more cross-arms, pins, insulators, and labor of erection. For a given effective voltage of transmission it is harder to insulate an alternating- than a continuous-current line. In the first place the maximum voltage of the alternating line with even a true sine curve of pressure is 1.4 times the nominal effective voltage, but the insulation must withstand the maximum pressure. Then comes the matter of resonance, which may carry the maximum voltage of an alternating circuit up to several times its normal amount, if the period of electrical vibration for that particular circuit should correspond to the frequency of the dynamos that operate it. Even where the vibration period of a transmission circuit and the frequency of its dynamos do not correspond, and good construction should always be planned for this lack of agreement, resonance may and often does increase the normal voltage of an alternating transmission by a large percentage. The alternating system of transmission must work at practically constant voltage whatever the state of its load, so that the normal stress on the insulation is always at its maximum. In a transmission with continuous current on the other hand, if the prevailing practice of a constant current and varying pressure on the line is followed, the insulation is subject to the highest voltage only at times of maximum load on the system. Lightning is a very real and pressing danger to machinery connected to long transmission lines, and this danger is much harder to guard against in an alternating system than in a system with continuous constant current. The large degree of exemption from damage by lightning enjoyed by series arc dynamos is well known, the magnet windings of such machines acting as an inductance that tends to keep lightning out of them. Moreover, with any continuous-current machines lightning arresters having large self-induction may be connected in circuit and form a most effective safeguard against lightning, but this plan is not practicable on alternating lines.