496 Prof. Fleming and Mr. Clinton on the 



strike a gong. Every hundred revolutions, therefore, of the 

 motor or commutator, the gong gives one stroke, and by 

 means of a stop-watch it is easy to take the time of ten 

 strokes of the gong : in other word.-, to ascertain the time in 

 seconds of a thousand revolutions of the motor, and therefore 

 of the number of commutations per second. In the case of 

 the motor we have employed. 1000 revolutions take place 

 generally in 40 seconds, which is at the rate of 1500 per 

 minute, and therefore corresponds with 100 commutations of 

 the condenser per second. 



We have tried various methods of making the rubbing 

 contacts and found nothing better than brass gauze brushes. 

 Carbon brushes were tried at one time and found to be 

 unsuccessful. It is essential that the commutator surface 

 should be kept bright and clean, and the brass gauze brushes 

 do this themselves when adjusted to the right pressure. 



Associated with this commutator we have emploved a gal- 

 vanometer of the movable coil type, either one made by 

 Crompton, or in some cases one made by Pitkin. Bv the aid 

 of this instrument, given a source of constant voltaoe by 

 which the motor can be driven steadily, such as a secondary 

 battery, the measurement of small capacities becomes an 

 exceedingly easy matter. 



There is. of course, no novelty in the mere use of a 

 rotating commutator for the determination of capacities by 

 the above method. It was employed many years ago by 

 Dr. P. T. Glazebrook for this purpose. (See ' The Elec- 

 trician.' vol. xxv. p. 616, 1890, on ''The Air Condensers of 

 the British Association.'" One of us (Dr. Flemmo-j had. 

 however, employed the device several years previously for 

 the same purpose. Our only claim to novelty in this matter 

 is that of having worked out a thoroughly satisfactory form 

 of rotating commutator, which is designed more from the 

 point of view of an engineer than an electrical instrument 

 maker. 



In the case of the measurement of capacity of insulated 

 wire> or aerials, the aerial is connected to the middle brush, 

 one terminal of the galvanometer and battery respectively 

 are connected to the two remaining brushes, and the other 

 terminals of the galvanometer and battery are connected to the 

 earth (seeng. 3). Under these circumstance.-, when the commu- 

 tator is in rotation, the galvanometer gives a perfectly steady 

 deflexion due to the passage through it of 100 discharges per 

 second from the condenser. In order to determine the 

 numerical value of the capacity, we have therefore to 

 evaluate the deflexion of the galvanometer and to determine 



