28o 



THE POPULAR SCIENCE MONTHLY. 



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 numer- 

 ous voyages to the islands of the whole Mar- 

 shall group, and had charts of them, which 

 were drawn and copied on sticks and stones. 



ImproTeuiruts ia Electro-Motors and 

 Dynanio-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. Dy- 

 namo-generators are efficient only when 

 their field-magnets are able to retain at all 

 times a certain amount of residual magnet- 

 ism. 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 dvnamo- 

 machines with soft-iron cores have met with 

 success, and, as electro-motors to give the 

 best results should have such cores, ma- 

 chines 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 cur- 

 rent theory of their action, viz., that the 

 electricity is generated by the inductive 

 action and reaction between the field-mag- 

 nets and the armature, is inadequate. Even 

 wrought iron contains some residual mag- 

 netism, 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. "Weisendan- 

 ger 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 in- 



definitely increased by augmenting that of 

 the field-magnets. This is characterized as 

 a mischievous theory whose outcome is per- 

 petual motion. The author, on the con- 

 trary, holds that there is a definite relation 

 between the power of the field-magnets and 

 the armature, which has yet to be experi- 

 mentally determined. Assuming the rela- 

 tion 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 re- 

 lating 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 concen- 

 tric. 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 arma- 

 ture reaches the magnet, the distance be- 

 tween the two remains constant while they 

 are passing each other. Mr. AVeisendangcr 

 holds that in generators the strongest cur- 

 rents 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 en- 

 tire 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 cither ap- 

 proaching or receding from the field-mag- 

 nets. 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 ser- 

 vices now rendered by combustion. He 



