498 Prof. Fleming and Mr. Clinton on the 



electromotive force o£ 100 volts or more between the coils 

 the resulting unavoidable leakage entirely vitiates the result. 

 We have therefore devised a differential movable coil gal- 

 vanometer which has been made for us by Messrs. James 

 Pitkin & Co.. and is constructed as follows : — 



In this galvanometer there are two sets of fixed field- 

 magnets, and also two movable galvanometer-coils completely 

 insulated from one another, but attached to the same stem, 

 which also carries the mirror. Very fine spiral flexible wires 

 convey the currents into and out of each coil. In order to 

 make the galvanometer differential and therefore show no 

 deflexion when the same current is passed in opposite directions 

 through the coils, it is necessary to be able to adjust exactly 

 the field-strength in the air-gap of the fixed magnets. This we 

 accomplish by means of two curved pieces of soft iron P, 

 which are moved by screws to or from the field-magnets N 3 S 

 (PL XII. fig. 4) so as to shunt more or less of the lines of 

 flux passing between the pole-pieces of the magnet. In this 

 manner we find we can construct a movable coil differential 

 galvanometer which shows no deflexion when the same or 

 equal currents are passed in opposite directions through the 

 two coils, yet each coil is perfectly insulated from the other. 



Employing such a differential movable coil galvanometer 

 in connexion with a commutator, we get rid of all necessitv 

 for measuring any voltage or electromotive force, and reduce 

 the measurement of capacity simply to a determination of the 

 speed of the commutator (wdiich can be taken with great 

 accuracy by means of a stop-watch) and the known value of 

 the shunt and series resistances in connexion with one coil 

 of the galvanometer. Moreover, we can always tell from the 

 speed of the commutator exactly the time during which the 

 condenser is in connexion with the galvanometer, and hence 

 whether the time of contact is, as it should be, large compared 

 with the time-constant of the discharge circuit. 



We have employed one or other of these methods in making 

 a number of measurements of the capacity of aerial wires, 

 such as are used in connexion with Hertzian Wave Tele- 

 graphy, and also in the investigation of the laws governing the 

 capacity of such wires when grouped together in certain 

 ways, and w^e have employed the arrangements for verifying 

 experimentally, as far as possible, the formulas that have 

 been given for the capacity of insulated wires in various 

 positions in regard to the earth. Taking first the case of 

 single vertical wires insulated in the air, measurements have 

 been made of the capacity of wires suspended vertically in 

 the open air, and also in the interior of a large Laboratory, 



