In running along walls all wires shall be rigidly attached to the same by nonconducting fastenings and shall not hang from projecting insulators in loops. All arc light wires shall be placed not less than one foot apart and incandescent light wires not less than six inches apart. The use of porcelain knobs or insulators on the outside of buildings is prohibited except in dry places, when an improved insulator must be used on the walls. (Code 1941, Art. 105-16)