44 Dr. E. H. Cook on Experiments 



Induction- Coil : — 



Length of primary wire . 200 feet (500 turns) . 

 Length of secondary wire . 60,000 feet (490,000 turns). 

 Capacity of condenser . . 2 microfarads. 

 Length of spark in air . . 0*07 metre. 

 Corresponding to 

 * Potential-difference of about . 63,000 volts. 

 Force from positive brush equal to a weight of 



0*01 gramme. 

 Force from negative brush equal to a weight 

 of 0'01 gramme. 



It will thus be seen that the magnitude of the force is 

 greater from the positive side of the machine than from the 

 negative. 



An attempt was now made to find the maximum distance at 

 which this mechanical disturbance could make itself felt. 

 For this purpose the following experiment was arranged. A 

 single fibre of unspun silk was stretched across the field of 

 view of the microscope, and on this was hung a little paper 

 index which was blown aside by the wind from the point. 

 The maximum distance at which any deflexion could be 

 observed was then noted, and in this way a comparison was 

 instituted. Care was again taken not to use different points, 

 but to alter the polarity by reversing the machine. Also, 

 during the experiment, the whole apparatus was carefully 

 protected from extraneous currents of air. It was found that 

 with the machine with a potential-difference of 33,000 volts 

 the positive brush produced an effect at a distance of 0'6 metre, 

 the negative at a distance of 0*48 metre. When the potential 

 difference had fallen to about 25,000 volts the distances 

 observed were for the positive 0*32 metre, and for the negative 

 028 metre. With the coil giving a spark of 4 centimetres 

 (41,500 volts) the positive brush affected the thread at a 

 distance of 32 metre, and the negative moved it at the 

 same distance. 



These results therefore confirm the former ; for the positive 

 brush affected the silk at the greater distance, and also pro- 

 duced the greater pressure upon the pan of the balance. But 

 they show how very quickly a moving electrified particle of 

 air is brought to rest by surrounding air. 



* These details are given in order that an idea may be formed of the 

 kind of apparatus worked with, and that if any one should desire to 

 repeat the experiments he may know what results to expect. The 

 actual numbers will vary with the apparatus. 



