The source of current was a Siemens’ alternating current dynamo of type _W_0, which was excited by a continuous current machine. The potential was supposed to be 1500 volts and the current 11·3 ampères. The main lead connecting the transformers in series was of 7 wires of 1·5 mm. diameter, and was 22·9 kilometres long, having a resistance of 30 ohms. Three stations were supplied. At Edgeware Road twelve coils, with their secondary coils in parallel, fed 30 glow lamps; and other four coils, also in parallel, fed two Jablochkoff candles. In Aldgate two coils supplied one arc lamp, and twelve more coils 35 glow lamps, each of 20 c.p. and three of 40 c.p. At Notting Hill there were 22 glow lamps and one arc lamp. In this last installation coils were employed with their coils arranged after a somewhat different manner. On a pasteboard or wooden cylinder of about 50 cm. in height a cable was coiled in layers. The interior of this cable consisted of a 4 mm. copper wire well insulated with paraffined cotton, and around this, parallel to its axis, lay 6 cables or cords, each consisting of 12 wires, also insulated with paraffined cotton (Fig. 21). The wire of 4 mm. formed the inductor through which the primary current was passed. The six cables, each of twelve strands, formed the induced portion of the apparatus, and the ends were connected to a commutator, so that they could be used either in parallel or series. [Illustration: FIG. 21.] The methods of construction and connection used in these attempts by Gaulard and Gibbs did not differ in principle from those of their predecessors. Gaulard and Gibbs also employed in these trials bi-polar induction apparatus. The efficiency of such apparatus can only be comparatively small, because the effects of magnetisation, and therefore of induction, are weakened to a great extent by the lines of force having to pass for the greatest part of their path through air instead of iron. Taking another view of such apparatus, as they have a ratio of transformation of 1:1, they must, with the employment of high potential, be connected in series. Undoubtedly Messrs. Gaulard and Gibbs have in their time claimed certain things as new and of their own invention, namely, the arrangement of several separate induction coils together, the placing of the coils next to one another, and the winding of the wires parallel. These claims, however, have been condemned from all sides as unjustified. The employment of several coils has already been mentioned as patented by the brothers Bright on 21st October, 1852, and was again later on discovered by Poggendorf, Ruhmkorff, Foucault, and others. We have also shown, on page 11, that the placing of the coils next one another had likewise been invented by the same men 30 years earlier. The symmetrical arrangement of both coils, the primary and secondary, had also already been used. (See page 18.)