iterial of Target, 



Lead 



(A) 



" 



(B) 



Iron 



(A) 



" 



(B) 



Zinc 



(A) 



Copper (A) 



Brass 



(B) 



of magnitude of the effect when a 1-gram bullet strikes a 

 metal target with a velocity of about 700 ft. per second is 

 six E.S. units, and approximately the same for + ve and — ve 

 ions, i.e., one E.S. unit per 3 x 10 7 ergs of kinetic energy 

 destroyed. Little variation was produced by varying the 

 material of the target so long as this is of metal. This is 

 shown by the following table of measurements : — 



Reading. 



16 ± 3 



69 + 4 



35 + 3 

 60 ± 3 



36 + 3 

 34 

 63 



Two sets of observations (A) and (B) are tabulated, the sensi- 

 bility of the electrometer being higher in the latter set. The 

 lead target used in (A) had been battered by previous usage. 

 that in (B) was a fresh plate. This is the probable cause of 

 the anomalous behaviour of the lead in the first case. When 

 a wooden target was used the deflection of the electrometer 

 needle was very much smaller than in the above cases. 



In the above cases the target was not perforated by the 

 bullet. When targets of lead-foil or sheet -lead were used 

 the effect was found to vary with the thickness of the target, 

 increasing with the thickness, but reaching a certain maximum 

 value, while the target was still easily perforable. This is 

 exhibited in the curve, ABC, shown in fig. 2. Other metals 

 were found to behave similarly as shown in the curve, DEC, 

 given for sheet-copper, the maximum ionization being the 

 same in all cases, but the critical thickness decreasing with 

 increasing density of the metal. The curve, FGC, is for 

 sheet-zinc. 



When the brillet perforates a target of sheet-metal the 

 air in front of the target is found to be ionized, but prac- 

 tically no ionization could be detected in the air behind the 

 target. For example, with the electrode and chamber in 

 front a reading of 34 scale divs. was obtained, whereas the 

 reading when both were transferred to the rear was only 

 0*32 scale divs. 



If two perforable targets were placed in series the total 

 ionization was found to be equal to the sum of the ioniza- 

 tions produced by each separately, provided the front target 

 is not allowed to act as an electrostatic shield to the rear one. 



