E. If we halve the length of the rubber strands, keeping the number of strands the same, the torque is but slightly increased for the first 100 turns; at 240 turns it is double. But the greater number of turns—in ratio of about 2:1—that can be given the longer strand much more than compensates for this.
F. No arrangement of the strands, per se, gets more energy (more motive power) out of them than any other, but there are special reasons for making the strands—
G. As long and as few in number as possible.
1. More turns can be given it.
2. It gives a far more even torque. Twelve strands 2 ft. 6 in. long give practically a line of small constant angle. Thirty-six strands same length a much steeper angle, with considerable variations.
A very good result, which the writer has verified in practice, paying due regard to both propeller and motor, is to make—
H. The length of the rubber strands twice[13] in feet the number of the strands in inches,[14] e.g., if the number of strands is 12 their length should be 2 ft., if 18, 3 ft., and so on.
§ 7. Experiments with 32 to 38 strands 2 ft. 6 in. long give a torque curve almost precisely similar to that obtained from experiments made with flat spiral steel springs, similar to those used in watches and clocks; and, as we know, the torque given by such springs is very uneven, and has to be equalised by use of a fusee, or some such device. In the case of such springs it must not be forgotten that the turning moment (unwinding tendency) is NOT proportional to the amount of winding up, this being true only in the "balance" springs of watches, etc., where both ends of the spring are rigidly fastened.
In the case of Spring Motors.[15]
I. The turning moment (unwinding tendency) is proportional to the difference between the angle of winding and yielding, proportional to the moment of inertia of its section, i.e., to the breadth and the cube of its thickness, also proportional to the modulus of elasticity of the substance used, and inversely proportional to the length of the strip.