Fig. 12.—Hand-wheel Arrangement for Working Dynamo.
B, Endless band passing round pulley of dynamo. H, Handle of wheel. S S, Handles attached to binding-screws of dynamo.
As a concluding illustration, I give a diagram of my own method of turning my dynamo (Fig. 12). On the leg of an ordinary table T is fixed the heavy iron wheel W, which has a groove cut in its circumference for the reception of an endless band B. These wheels may be obtained for a few shillings from any ironmonger, as they are made for various machines, such as laths, fret-saws, sewing-machines, etc. The wheel is held by an ordinary screw fixed into the leg of the table, and revolves on the screw. The endless band (tape will do) passes over the groove and over the pulley of the dynamo placed on the table above the wheel.
It is better to let the pulley of the dynamo project beyond the end of the base, as shown in Fig. 11, in order to be able to connect it with a wheel placed below it, if required.
The best results are produced from the dynamo when the resistance of the interpolar (i. e. the lamp, or whatever it may have to work) is equal to the internal resistance of the machine. It is sometimes required to send a current through a greater resistance than this, and then it becomes necessary to employ what is familiarly termed a “shunt.” If one lamp of high resistance is coupled to the dynamo, the resistance may be too great for the current to get round the magnet in sufficient quantity to give the required electromotive force. Supposing that this is the case, we make a second pathway for it by joining on a piece of iron wire (about ten inches of No. 30) between the two binding-screws, the lamp being connected with the same binding-screws, only further off. The result of this is that the current goes round by the second pathway and excites the magnet more powerfully, and this, in its turn, excites the armature more strongly, and so on, until enough current is produced to light up the lamp. The resistance of the shunt required depends on the resistance of the lamp. If this is low no shunt will be required, if very high the resistance of the shunt must be lowered, or else enough current will not pass to magnetize the soft iron cores, and the dynamo will give no current. The lower the resistance of the shunt required, the less wire we use.