r 



TRANSACTIONS OF SECTION G. 811 



8ist of a few turns only of forged copper bars, oue-aud-a-lialf inches wide by one 

 inch thicii, forged to fit the magnet cores. There is no insulation other than mica 

 wedges to keep the bars from touching the core. 



The armature is ventilated by a current of air from a Schiele's fan, driven by a 

 belt from the armature spindle. At 380 revolutions the armature can give 5,000 

 amperes at 60 volts, and the temperature of the hottest part, which is the end 

 plates next the commutator, never rises beyond 70° Cent. Although this machine 

 was designed to give only 5,000 amperes, yet, electrically speaking, it is capable of 

 doing very much more, and in actual practice it has frequently to stand a current 

 of 8,000 amperes for short periods. But, as will be shown later, when the furnaces 

 are employed to produce ferro-aluminium there are frequently violent oscillations 

 in the current, which would make it unsafe to employ an average current much in 

 excess of 5,000. In fact, as has been stated above, it has been found necessary 

 already to increase the number of driving-teeth in order to provide against the 

 driving strains at the times of maximum current. When the machine was first 

 tried the main circuit was unprovided witli a cut-out, and there is no doubt that at 

 this time the armature stood the test more than once of temporary currents exceed- 

 ing 16,000 amperes. Since that time a safety cut-out has been provided, calculated 

 to fuse at 8,000 amperes ; this, probably tlie largest one that has ever been de- 

 signed, consists of a framework carrying twelve lead plates, and these fuse at the 

 calculated point in a most satisfactorj' manner, without any undue detonation or 

 scattering of the molten lead. 



Next to the cut-out comes the current indicator, which is simply a solenoid 

 of nine turns through which the whole current passes. The core is attached by 

 chain-gear to the pointers, of which there are two mounted on one spindle. One 

 pointer and dial is placed in the engine-room, and the other in the furnace-room. 

 The gearing of this spindle is calculated so that the range of the instrument up to 

 8,000 amperes extends over the entire circle of 360 degrees. The construction of 

 the solenoid for this indicator is somewhat novel ; it is cut out of a cylinder of cast 

 copper by means of a parting tool in a screw-cutting lathe. 



There are two furnace-rooms, each containing six furnaces, and the conductors, 

 after leaving the current indicator, pass right across the two rooms at a height 

 somewhat above the attendants' heads. The current is supplied to the furnaces aa- 

 follows : — 



Each furnace-pit consists of a long trough built of firebrick, the ends being 

 closed by cast-iron pipes through which carbon electrodes are arranged so that 

 they can be moved lengthways to and fro by the attendant in charge. It will be 

 remembered that one of the main features of the Cowles' invention is the use of 

 limed charcoal as a lining for their furnace ; they thus obtain a highly refractory 

 non-oxidising lining which is also a non-conductor of electricity, as the lime pre- 

 vents the particles of carbon fusing together and thus forming a continuous con- 

 ductor, which would otherwise short circuit the furnace and be consumed under 

 the action of the current, instead of the latter performing its proper duty of main- 

 taining the furnace contents at the required high temperature. The electrodes — 

 each of which consists of nine carbon rods attached to a cast-iron head, mounted 

 on a copper rod passing through the above-mentioned cast-iron pipes — are each 

 provided with flexible copper-wire cable connections, having an attachment sliding 

 along the conductors passing overhead and somewhat behind them. When it is 

 desu-ed to supply the current to any one furnace, the attachments are slid into posi- 

 tion opposite it and clamped fast to the conductors ; to start the furnace the elec- 

 trodes are brought together so as to touch, and then separated until the whole of 

 the contents of the furnace between them is an incandescent mass at the required 

 high temperature. 



The working of a charge in the furnace is carried out as follows : the bottom 

 of the empty furnace is covered w^ith limed charcoal ; the electrodes are brought 

 into position, and a former, or iron box without top or bottom, and of a size deter- 

 mined by the space to be occupied by the charge, is placed in the furnace ; the 

 ends of this sheet-iron box are arched out so that it can be dropped over the elec- 

 trodes ; the furnace lining of limed charcoal is then rammed up iu the space 



