TRANSACTIONS OF SECTION A. 501 



sp^ciale, nous avona pu obtenir des imaoes solaires ou une partie est positive, une 

 partie neutre-claire, une partie negative du deuxieme ordre, etc. 

 J'ai I'honneur de joindre a cette note : 



1° Une image solaire de 10 centimetres (boite)de diametre, positive. 

 2" Une image solaire de 4 centimetres de diametre, positive. 

 .3" Une image solaire avee partie neutre-claire. 



4° Une image solaire ayant une partie positive, une neutre-claire, une negative 

 du deuxieme ordre, etc, 



5° Un paysage n(5gatif avec soleil positif dans un ciel negatif. 

 C^ Un paysage coupe en trois parties, obtenu par contact avec un cliche 

 ii^gatif. 



Premier tiers : negatif l''^ ordre. 

 Deuxieme tiers : positif. 



Troisieme tiers: negatif, 2'"° ordre (les apparences sont inverses, parce 

 que le cliche producteur est negatif). 

 7° Une photographie de taches solaires obtenue d'un cliche de 30 centimetres de 

 diametre pour un grossissement de trois fois. Cette photographie montre les stries 

 de la p^nombre et les granidations de la surface environnante. 



10. On Improvements in Electro-Motors} By Theodor Wiesendanger. 



1. The inventors of the most recent electro-motive engines have worked — 

 perhaps unconscioush' — upon the idea, that the construction and action of electro- 

 motors are based altogether upon the same laws as those of dynamo and magneto- 

 machines and in accordance with that assumption the field-magnets of the Desprez- 

 Motor are made to consist of large and heavy masses of magnetised steel. 



2. Experimenters have also for a long time past clung to the idea that the 

 efficiency of an electro-motor, or the amount of energy to be obtained from such 

 a machine, by means of a current of given strength circulating in the coils of its 

 armature only, bears a definite and direct proportion to the magneto-inductive 

 power of its field-magnets, and that an increase of power in the field-magnets 

 alone must necessarily produce greater capabilities of the machine. 



3. This, however, is a mischievous theory, because erroueous in its very 

 principles, and its development would only lead to the hypothesis of perpetual 

 motion. On the contrary, starting from a consideration of the facts that a very 

 small magnetic needle, if acted upon by one of the poles of another and very 

 powerful magnet, has its polarity destroyed or reversed, and that, if one of its 

 poles, say the N pole, is presented to a similar (N) pole of the large magnet, the 

 former will completely lose its characteristic qualities and be attracted by its over- 

 powering opponent, we can only come to the one rational conclusion, that the 

 power of the field-magnets of an electro-motor, as compared with that of the magnet 

 or magnets constituting its armature, should not surpass the limit of some certain 

 ratio, yet to be determined by experiments carefully conducted, and that, if it sur- 

 passes that limit, the capabilities of the machine must be impaired. Acting on 

 this principle the inventor constructed a motor (the motor was shown in motion) in 

 which the power of the field-magnets is as nearly as possible equal to that of the 

 armature, the core of the former being very light and made entirely of soft iron; 

 and the satisfactory' results obtained from this machine are a sure sign that a 

 farther investigation of the subject, and experiments made with a view of deter- 

 mining the exact ratio, will lead to further improvement. Another verj' important 

 consideration in the construction of electro-motors is the method of motion of 

 the revolving armature, with regard to the approach to, or the receding of its 

 poles from the poles of the field magnets. The greatest amount of power will 

 be derived from a motor if attention is paid, not merely to the one condition 

 that the armature should revohe in the most highly concentrated magnetic 



' Published in- extcnuo, with illustrations, in the English Mechanic ; also in 

 Design and Wirrli, September 18, 1880. 



