Page:Popular Science Monthly Volume 18.djvu/292

280 Mats and hats are artfully woven out of the bark of a shrub called the loa, and colored in handsome patterns of yellow, red, and black. The natives formerly made numerous voyages to the islands of the whole Marshall group, and had charts of them, which were drawn and copied on sticks and stones.

Improvements in Electro-Motors and Dynamo-Machines.—In a paper recently read before the British Association, Mr. T. Weisendanger takes exception to some of the received theories regarding electro-motors and dynamo-generators, and points out an improved mode of construction for both. In regard to the relations of these two classes of machines, it has generally been held that the most efficient generator is also the most efficient motor. This Mr. Weisendanger considers erroneous. Dynamo-generators are efficient only when their field-magnets are able to retain at all times a certain amount of residual magnetism. Their cores are, therefore, usually made of hard cast iron, or, if of soft iron, they are attached to masses of cast iron so that these form part of them. None of the efforts hitherto made to construct dynamo machines with soft-iron cores have met with success, and, as electro-motors to give the best results should have such cores, machines can not be made that will give the maximum efficiency in both kinds of work. The fact that the attempts to make dynamo machines with soft-iron cores have resulted in failure, he considers, proves that the current theory of their action, viz., that the electricity is generated by the inductive action and reaction between the field-magnets and the armature, is inadequate. Even wrought iron contains some residual magnetism, and in large masses, and after it has been subjected to strong magnetization, the amount is considerable. By the theory, the smallest amount of such magnetism would be sufficient to start the action of the machine. Experiment, however, shows that this is not the case. Mr. Weisendanger does not offer a new theory, but insists that the present one needs to be amended to correctly express the facts. Attention is also called to the idea underlying the work of some recent experimenters, that the power of an electro-motor can be indefinitely increased by augmenting that of the field-magnets. This is characterized as a mischievous theory whose outcome is perpetual motion. The author, on the contrary, holds that there is a definite relation between the power of the field-magnets and the armature, which has yet to be experimentally determined. Assuming the relation of these sets of magnets to be one of equality, he has constructed a motor, in which the cores of the field-magnets are light pieces of soft iron, that gives very satisfactory results. Further experiments to determine the exact ratio of the power of the field and armature, he believes, will result in a much more perfect machine. The most novel and perhaps important part of Sir. Weisendanger's paper is that relating to the proper method of revolving the armature before the poles of the field magnets. The present practice is to make the cores of the field-magnets and those of the armature of such shape that the circles in whose circumference they lie are concentric. The defect of this arrangement is, that the armatures approach the magnets through the space in which the intensity of the field is at a minimum. After the armature reaches the magnet, the distance between the two remains constant while they are passing each other. Mr. Weisendanger holds that in generators the strongest currents will be induced, and in motors the greatest amount of power obtained when the armature not only revolves in the most highly concentrated field, but when its entire motion is either one of approach to or withdrawal from the field-magnets. He, therefore, proposes that the field-magnets be set at an angle to the circle described by the revolving armature. This latter then approaches the former continuously to the very instant of its leaving them. The greater the number of magnets the more powerful the action, as the armature is throughout its entire movement either approaching or receding from the field-magnets. Mr. Weisendanger is very hopeful of the future possibilities of electricity. Our present machines he believes to be but very imperfect appliances, which further research may so improve that the electric current will eventually perform all the services now rendered by combustion. He