Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Ilhistrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew. M.A. 

 APRIL, 1912. 



A SIMPLE RECORDING DE\'1CE EOR ATMOSPHERIC 



ELECTRICITY. 



Bv CHARLES E. BENHANL 



J' 



Figure Xi 



A SIMI'LE form of recording instrument for use 

 with any apparatus that collects atmospheric elec- 

 tricity is of great assistance in stud\ing the subject. 

 The form of collector may be either the water- 

 dropping apparatus invented by Lord Kelvin, or 

 preferably the more recent col- 

 lector patented b\- Mr. F. H. 

 Glew, in which the ionisation 

 properties of radio-actives are 

 ingeniouslymade use of to enable 

 the collecting rod to take up the 



charges from the atmosphere. The wire from either 

 of these forms of collector mav be led through a 

 carefully insulated plug of sul[)hur into a room to 

 affect a gold leaf electroscope there, and by the 

 attachment here to be 

 described the charges mav 

 be made to register them- 

 selves automatically. y^ — — 



The first essential is a y^ 

 sort of relay which will 

 operate whenever the 

 atmospheric electrification 

 rises to a given potential. 

 A balance of light wire is 

 made in the form shown 

 in Figure 132. about six 

 inches in length, the ex- 

 tremities being brought 



just below the level of the Figurf. 133 



axis, so as to ensure stable 



equilibrium and considerable sensitiveness. One 

 end terminates in a plate of copper foil about two 

 inches square. The other end is soldered to a fine 



tROO»fO ^^^p 



steel needle attached vertically. The axes are two 

 fine points, which rest in a grocjved strip of brass, as 

 shown in Figure 133. A counterbalance on the wire 

 is adjusted till the balance will rest horizontally. 

 Underneath the end bearing the steel needle is a 

 small cup of mercury, the needle 

 \ , „ , coii- point being poised about one- 



."" sixteenth of an inch above the 



surface. Under the copper foil 

 at the other end is a metal plate 

 of the same size, insulated and 

 connected w ith the atmospheric collector. It should 

 be about one-eighth of an inch below the copjier foil. 

 It is evident that anv electrification of the metal plate 

 will attract the copper foil, and if powerful enough will 

 draw it down until it 

 touches and discharges the 

 lower plate, when it will re- 

 bound and cause the needle 

 at the other end to dip into 

 the mercury cup. Wires 

 are attached to this cup 

 and to the central brass 

 strip supporting the axis of 

 the balance, and these wires 

 communicate with a dry 

 cell and an electro-magnet, 

 which is thus brought 

 into action every time the 

 balance discharges the 

 electricity of the collector. 

 The electro-magnet acts upon an armature that 

 is fixed into a small block of wood mounted on the 

 end of a flat spring about four inches long, so that 



