528 SCIENCE PROGRESS 



a working knowledge of the behaviour of electricity in motion 

 and at rest. It has one obvious characteristic, that the greatest 

 difficulties are met with at the beginning. 



The alternative method designed for young boys will not be 

 described at length, as it has been adopted in some schools 

 during several years past and is dealt with in detail in text- 

 books. But as criticisms have been made of the treatment of 

 Potential and Ohm's Law, an outline of the presentation of this 

 part of the subject may be given. No knowledge of electro- 

 statics is assumed ; the course begins with experiments on cells, 

 galvanometers, etc., which give some general ideas of current, 

 resistance and electromotive force. A " legal " ohm is defined 

 and the definition may be extended to the effect that n ohm coils 

 in series have a resistance of n ohms. Copies are made by 

 substitution for the standard in a circuit containing a battery 

 and galvanometer. Although it could easily be shown here that 

 a copy accurate for one current is accurate for all but that it 

 must be at a definite temperature, it seems better to defer these 

 points till Ohm's Law is reached. Next, the legal (electrolytic) 

 unit of current is defined and a moving-coil ammeter thereby 

 graduated or calibrated ; the mechanics required for the com- 

 prehension of this instrument is regarded by the unspoilt mind 

 as mere common sense. 



Next comes the definition of the legal unit of electromotive 

 force, based on the standard cell ; previous experiments with 

 batteries have instilled a belief that potential differences are 

 additive, so moving-coil voltmeters of high resistance are 

 calibrated or graduated on this assumption ; as the calibration 

 curve is found to be a straight line, interpolation is obviously 

 permissible between the positions which the pointer takes up 

 for successive additions of a standard cell to the battery used 

 in graduating the instrument. Thus the units of resistance, 

 current and electromotive force are defined independently and 

 in such a form as to be at once applicable to numerical results. 

 By considering a battery sending a current through a series of 

 resistances, the boy gets a conception of the potential differ- 

 ences between the ends of a conductor as that fraction of the 

 electromotive force of the battery which is employed in driving 

 the current through the conductor. The relation between 

 current, potential difference and resistance is then determined 

 by experiments on conductors carrying a current furnished by a 



