The lower illustration gives a longitudinal and transverse section of the 'G-B.' system of surface-contact tramway traction. The rope-like cable carries the current and is supported on insulators. When the collector on the car covers the stud, the action of the magnet draws the lower part of the stud into contact with the cable, thus supplying current to the car. After the car has passed, the lower part of the stud rises by the action of a spring and, breaking contact with the 'live' cable, becomes dead. (In actual practice contact would be made under the conditions shown in the left-hand diagram.)] Experience of these drawbacks led the London County Council to seek an alternative to the conduit when constructing electric lines in the north of London. Many of the borough councils, following the County Council's own previous arguments, would not listen to the suggestion of the overhead system; and a freshly-elected Council, pledged to a policy of economy, determined to try the surface-contact system. How this trial gave rise to a violent political controversy, leading to the abandonment of the project and culminating in important libel actions, forms a picturesque story which need not be told in detail here. Its main interest lies, for the moment, in the emphasis which the incidents give to a characteristic of the surface-contact system--its sensitiveness to minute alterations in detail. The surface-contact or 'stud' system is really a modification of the conduit system. It has, in fact, been called the 'closed conduit.' The electric wires are again placed in a channel or pipe underground, but instead of being accessible through a slot, contact can be made with them only through metal studs placed at intervals flush with the roadway. By special electro-mechanical devices in the stud and on the car, the stud is brought into contact with the 'live' underground wire only when the car is over it. That is to say, the studs covered and protected by the car will be 'live' and supplying power to the car through a sliding brush or 'skate,' while those not so protected will be 'dead' and therefore of no danger to the public.