378 



SCIENCE. 



[N. S. Vol. IX. No. 219. 



Measures and the great woi'k it has accomplished 

 during the past twenty-five years. Nothing is 

 said about the International Prototype units of 

 length and mass, which are the real standards 

 of the world to-day, and it is implied that the 

 meter and kilogramme are, except for practical 

 purposes, what they were defined to be a hun- 

 dred years ago. The ratios of the metric to our 

 customary units used, in the book, are not those 

 legally adopted by the U. S.Ofiice of Weights and 

 Measures, but the differences are so small that 

 the conversion tables are not sensibly in error. 

 There is a growing use of the metric system in 

 this country, the result of an increasing trade 

 with foreign countries, and this book will satisfy 

 every demand of those who are called upon to 

 convert from one system to the other. The 

 tables are so numerous that it is difficult to 

 imagine a call for anything which the book does 

 not contain, and a convenient index renders 

 them quickly available. Much time is saved 

 by carrying the tables up to one-hundred mul- 

 tiples of each unit, but in a few instances space 

 and labor have been wasted in doing this, be- 

 cause of the impossibility of such conditions 

 ever being realized. For instance, in the table 

 for converting ' grammes in a cubic centimeter 

 to ounces in a cubic inch,' there does not appear 

 to be any necessity for going beyond 23 or 24 — 

 as there is no known substance denser than this. 

 Thus more than three-quarters of this table can 

 be of no use, and this is true of several tables 

 of a similar character. On the whole the work 

 is exceedingly well done, and the book ought 

 to be much sought after. T. C. M. 



THE ELECTROLYTIC INTERRUPTER FOR THE 

 INDUCTION COIL. 



When a high electro-motive force is connected 

 to an electrolytic cell, one electrode of which is 

 very small, the rush of current which takes 

 place is quickly interrupted by the layer of gas 

 which is generated at the small electrode. 

 This layer of gas then collects as a bubble, the 

 electrolyte again comes into contact with the 

 electrode, a rush of current again takes place to 

 be interrupted as before, and so on. These in- 

 terruptions are very abrupt, and their frequency 

 varies from two or three hundred to a thousand 

 or more per second according to the size of the 



small electrode and the inductance of the cir- 

 cuit. The small electrode should be the anode. 



Dr. A. Wehnelt {Eleclrical Engineer, February 

 16, 1899) has applied this electrolytic inter- 

 rupter to the induction coil. He uses dilute 

 sulphuric acid, a sheet of lead as cathode, and 

 the tip of a small platinum wire projecting from 

 a glass tube as anode. The interrupter works 

 with entire satisfaction with electro-motive 

 forces as high as 110 volts ; the condenser, 

 needed with the ordinary interrupter, is use- 

 less ; and the effectiveness, especially of small 

 coils, is greatly increased both in length of 

 spark and frequency. 



Dr. Wehnelt's experiments have been re- 

 peated in the Physical Laboratory at Bethlehem 

 Pa., his results have been confirmed, and it has 

 been found that the primary of an induction 

 coil should be wound with more turns of wire 

 than usual to give the best results with this 

 electrolytic interrupter. The interrupter gives 

 good effects when used to supply intermittent 

 current to the primary of a small transformer. 

 Thus a small step-down transformer taking 375 

 watts from the mains gave out about 30 watts 

 from its secondary. 



When the electrolytic interrupter is used to 

 supply intermittent current from a 110 volt 

 source to the primary of a transformer, the 

 e. m. f. which establishes the current after each 

 break is, of course, 110 volts, while the e. m. f. 

 which stops the current is the e. m. f. between 

 the break points and may be very greatly in 

 excess of 110 volts. 



The effective primary e. m. f. is, therefore, 

 on the whole, greatly in excess of 110 volts, so 

 that a 1:1 transformer may give several hun- 

 dreds of volts at its secondary terminals when 

 supplied with intermittent current from a 110 

 volt source. 



This is shown by the fact that a 220 volt 

 lamp, for example, may be lighted from the 

 secondary, and, of course, it may be lighted 

 equally well or even better if connected across 

 the primary terminals. W. S. F. 



THE RESISTANCE OF CARBON AND COPPER 

 BRUSHES. 



Professor E. Arnold gives, in the Electrical 

 Zeitschrift for January 5th, a study of the ' Con- 



