The best practical result is obtained when their resistance together with that of the lamp is equal to the total internal resistance of the dynamo machine. Wires of various sizes are therefore required. Decrease in strength of the current caused by a leading wire of too high resistance can be overcome by a higher velocity, which is obtained only by increased motive power, but if the wire is much too small, it will become heated. The proper remedy is to increase the sectional area of the leading wire. Bright sparks should never be allowed to appear at the commutator and brushes, as sparks result from a rapid burning of the metallic parts. They can easily be avoided by properly inclining the two arms which carry the brushes. The position of the brushes yielding the least spark at the commutator is that giving the highest intensity of light in the electric arc. The commutator should, while in motion, be freely oiled, to prevent the brushes wearing away too rapidly. The sticky oil should from time to time be removed by washing with paraffine oil or benzoline. _Wear and Tear._--The chances of stoppage so common to the old forms of electric light apparatus have in this form been reduced to a minimum, and now do not exceed those that arise with machines generally. The Trinity House Report states that the Siemens' machine worked well for a month without any necessity for stopping. The brushes are the only parts which wear away, and they are very easily replaced. In thick weather they should be connected in what is called parallel circuit (or parallel arc, or for "quantity"), because it has been found that when they are so arranged the intensity of the electric light produced exceeds by some twenty per cent. the intensity of the sum of the two when worked separately. Thus the two machines, giving respectively a candle power of 4,446 and 6,563 when worked separately (total 11,009), have given when coupled up in parallel circuit a light equivalent to 13,179 candles; just as in telegraphy it has been found that the rate of sending can be increased from 20 to 25 per cent. when the apparatus is coupled up in parallel arc. For this reason it is usual to employ two machines of medium size instead of one machine of large size. The intense light so produced is also much more uniform than from one large machine. _Automatic Electric Lamp._--Automatic electric lamps have been constructed with spring clockwork to cause the carbons to approach one another to a certain point, when, by means of an electro-magnet, the clockwork is checked, and the carbon points are allowed to burn away to such a distance that, by the decrease of current, the clockwork is released and the carbons caused to approach again. With such lamps the clockwork has been a source of trouble, and it is liable to get out of order.