68 Mr Palmer, Methods of Increasing the Spark- Length, etc. 



is not too great a spark passes, otherwise the brush only is 

 formed. 



The experiments made in various gases point to the same 

 conclusion. The phenomena of Expt. 1 were the most difficult 

 to be obtained in gases such as coal-gas and carbon-dioxide, which 

 only produce a brush with difficulty, while with such a gas as 

 nitrogen, where a brush was easily obtained, the effects of the 

 same experiment were easily produced. 



Faraday noticed that the spark-length of a machine was 

 always increased by having the smaller ball positive, and many 

 makers of machines arrange the terminals in this way. The 

 lengthening may be due to a positive brush being formed on that 

 terminal just previous to the discharge by spark, and the smaller 

 the ball the lower the potential at which the brush is produced. 

 If the smaller ball is negative a brush is formed there at a lower 

 potential, and this simply discharges the machine, aiding but 

 little in the passage of the spark. 



The results of Experiment 5 can also be explained by the 

 brushes produced. Whenever the glass rod is brought near 

 either terminal a slight crackling is heard due to the brush 

 discharge between the metal ball and the glass ; this, as before, 

 makes the passage of the spark easier, especially when the positive 

 brush is produced. 



With metals, whether insulated or not, flames, etc., the positive 

 brush produces discharge when they are held near the positive 

 terminal ; when near the negative a small spark passes and we 

 really have a repetition of Experiment 1. 



It is impossible to say why the effects are not so great when 

 the brush is formed on the negative ball until we explain the 

 difference between the positive and negative brushes ; their ap- 

 pearance has long been noted to be different and the above 

 experiments show that the presence of the positive in a gap 

 makes a gas much weaker electrically than if the negative were 

 present. In this action we have yet another difference between 

 the two electricities. 



All the above experiments were undertaken at the Cavendish 

 Laboratory with the exception of (5) and those with different gases 

 which were done at the Technical Schools, Southend. 



I take this opportunity of expressing my gratitude to Professor 

 J. J. Thomson for his kindness, and for the assistance he has given 

 me during the progress of these experiments. 



