of which there are four in use on all machines, are made of a broad strip of springy copper having six slits two thirds the distance up, and thus touching at several points. They are held by clamps shown at Fig. 23 which also shows the brushes. The brushes are held to the commutator by their own springiness and the variation of position due to strength of current. The brushes are set by a gauge sent with each dynamo which shows length from the end of brush to the holder. The holders are set at the correct angle by a gauge of brass of the shape of a right angled triangle the short side having a wide flange curved to fit the commutator for which it is sent, while the second side as regards length must fit to the holder when swung to it on the commutator as an axis.
After describing the details of the dynamo, we will at once proceed to find how the
Thomson Houston Dynamo Operates.
In the diagram Fig. 19 the rotation is as in practice against the hands of the watch when seen from the commutator end of shaft. The three coils of the armature are represented by three lines A, B, C, united at their inner extremities each being joined to a segment of the commutator. There are two positive brushes P and F and two negative ones P´ and F´. The current delivered to P and F goes round one of the field magnet coils, then to the outer circuit consisting of regulating gear, lamps, motors, etc., through the other field magnet coil to brushes P´ and F´. Now from Fig. 20, we observe that supposing the loop to be rotating against the hands of the watch in a magnetic field the diagram represents by arrows the direction of the electro-motive forces induced in those loops. The action is a maximum along the line of the resultant magnetic field m m´ and the minimum along the line n n´ which is at right angles to m m´. The reason that m m´ is not horizontal is that the induced poles of the armature is in advance of the poles of the field magnet and is constantly tending to be drawn back. Applying Fig. 20 to Fig. 19, we see that there will be an outward current in B, an inward one in C, A generating no current for that moment.
Now the following pair of brushes F F´ are shifted backward three times as far as P P´ is shifted forwards. When the current is the greatest possible the brushes P and F and P´ and F´ are 60° apart thus leaving P and F´ and P´ and F´ just 120° apart and since the segments of the commutator are each 120° in length[[3]] there will always be two coils in parralel with one another and in series with the third. Taking one sixth of a revolution and continuing all the way round we find the following tabulated statement showing brushes in contact with coils, to be true viz:—
| { P - C } { P´ } | ||
| From external circuit | { } B { } | to external circuit |
| { F - A } { F´ } | ||
| { P } { P´ - B } | ||
| ” “ ” | { } A { } | ” “ ” |
| { F } { F´ - C } | ||
| { P - A } { P´ } | ||
| ” “ ” | { } C { } | ” “ ” |
| { F - B } { F´ } | ||
| { P } { P´ - C } | ||
| ” “ ” | { } B { } | ” “ ” |
| { F } { F´ - A } | ||
| { P - B } { P´ } | ||
| ” “ ” | { } A { } | ” “ ” |
| { F - C } { F } | ||
| { P } { P´ - A } | ||
| ” “ ” | { } C { } | ” “ ” |
| { F } { F´ - B } |
Now suppose the current to become to strong owing to any cause, the following brushes are made to recede. This can but shorten the time that the brushes are in contact with the commutator when the coil is passing through that position in which it is generating the maximum amount of current and also hasten the time when it goes into parralel with a comparatively idle coil. If the current is to weak then the brushes are made to close up thus reducing the time that the most active coil is in parralel with one less active and also makes the brushes be longer in contact with the segment when the coil is generating its maximum amount of current. The motion of advance and retreat of the brushes is accomplished by the Thomson Regulating Gear before described. On Fig. 23 can be seen all the dynamo’s details except the Controller magnet.
Fig. 18.
Fig. 19.
Fig. 20.
Fig. 21.
Fig. 22.
As regards the Thomson-Houston Dynamo it will be found to produce the steadiest and most uniform current of any dynamo now in use. It regulating gear is the simplest and most natural one ever used. In its ability to reduce the current simaltaneously to one tenth of its former quantity inside of one or two minutes without injury to itself and lamps it stands alone, in practice.