86 Method of Measuring the Mutual Induction of two Coils. 



with sliding contacts. This circuit was made or broken by 

 the switch 0. A pilot wire, D, round the model electro- 

 magnet was connected in series with the secondary coil of B 

 and with the ordinary reflecting-galvanometer, Gr. The 

 arrangement was fairly sensitive ; but had one fault, which 

 was in fact foreseen. Instead of no current, the galvanometer 

 gets a current first one way and then the other, so that the 

 spot jerks each way. This might be remedied by another 

 arrangement of the wires ; but a similar difficulty will arise in 

 dynamo work. The flux of the induction through the standard 

 coil varies as the flux of the primary current ; but the flux of 

 the induction through, say, a section of a field-magnet depends 

 on the saturation of the iron. To avoid these difficulties, a 

 galvanometer with a heavy ballistic astatic needle of the kind 

 introduced by Ayrton and Perry will be used. This has been 

 made by Mr. Dobson, lately one of Messrs. Crompton's pupils. 

 It is of somewhat novel design. The coils, of which there are 

 several sets, can be changed without dismounting the needle, 

 so that the instrument can be used as a high or low resistance 

 or differential galvanometer ; and can be also used as a bal- 

 listic, or as a flywheel or integrating galvanometer. If 

 necessary, little arms with balls like those of a minute fly- 

 press will be added. The final apparatus was not put in hand 

 till quite lately, as there was no suitable room available for 

 such work ; but it is now being made. 



It was intended to calibrate the induction-coils from coils 

 whose mutual induction could be calculated ; or from a dy- 

 namo by measuring the induction through the armature with 

 a known exciting current, and then running the dynamo and 

 measuring its electromotive force and speed. This method 

 has, by the way, been recently mentioned by M. Kapp, for 

 calibrating a ballistic galvanometer. Prof. Foster's method 

 seems infinitely more convenient, and may save much trouble 

 if a good condenser is to be had. Of course an ammeter will 

 be in the primary circuit. 



It would seem that a flywheel-galvanometer might be used 

 for such purposes as finding the ohm from a pair of coils with 

 calculated mutual induction. Suppose, for instance, a contact- 

 breaker makes and breaks contact in the primary circuit with 

 a known frequency, and allows the impulses one way in the 

 secondary coil to go round one coil of a differentially- wound 

 flywheel-galvanometer. A constant current in the other coil 

 is regulated to oppose these impulses ; and from these the 

 ohm might be got. Either the contacts must be made slowly 

 enough for the current in the primary to come to its perma- 

 nent value, or a correction must be made for the error. There 



