The most satisfactory apparatus for electro-culture is that used in the Lodge-Newman method, designed by Sir Oliver Lodge and his son, working in conjunction with Mr. Newman. This consists of a large induction coil supplied with current from a dynamo driven by a small engine, or from the public mains if available. This coil is fitted with a spark gap, and the high-tension current goes through four or five vacuum valve globes, the invention of Sir Oliver Lodge, which permit the current to pass through them in one direction only. This is necessary because, as we saw in Chapter VIII., two opposite currents are induced in the secondary winding of the coil, one at the make and the other at the break of the primary circuit. Although the condenser fitted in the base of the coil suppresses to a great extent the current induced on making the circuit, still the current from the coil is not quite uni-directional, but it is made so by the vacuum rectifying valves. These are arranged to pass only the positive current, and this current is led to overhead wires out in the field to be electrified. Lemström used wires at a height of 18 inches from the ground, but these were very much in the way, and in the Lodge-Newman system the main wires are carried on large porcelain insulators fixed at the top of poles at a height of about 15 feet. This arrangement allows carting and all other agricultural operations to be carried on as usual. The poles are set round the field, about one to the acre, and from these main wires finer ones are carried across the field. These wires are placed about 30 feet apart, so that the whole field is covered by a network of wires. The electricity supplied to the wires is at a pressure of about 100,000 volts, and this is constantly being discharged into the air above the plants. It then passes through the plants, and so reaches the earth. This system may be applied also to plants growing in greenhouses, but owing to the confined space, and to the amount of metal about, in the shape of hot-water pipes and wires for supporting plants such as vines and cucumbers, it is difficult to make satisfactory arrangements to produce the discharge.