22 THOMSON. — ATMOSPHERIC ELECTRICITY. 



III. To use the portable electrometer for observing atmospheric 

 electricity : 



(1.) The place of observation, if on board ship, must be as far 

 removed from spars and rigging as possible. In a sailing ship or 

 rigged steamer the best position for the electrometer generally is 

 over the weather quarter when under way, or anywhere a few feet 

 above the tafferel when at anchor. On shore or on the ice a 

 position not less than 20 yards from any prominent object (such 

 as a hut or a rock or mass of ice or ship), standing up to any 

 considerable height above the general level, should be chosen. 

 Whether on board ship or in an open boat or on shore or on the 

 ice, the electrometer may be held by the observer in his left hand 

 while he is making an observation ; but a fixed stand, when con- 

 veniently to be had, is to be preferred, unless in the case of making 

 observations from an open boat. 



(2.) To make an observation in ordinary circumstances the 

 observer stands upright and holds or places the electrometer in a 

 position about five feet above the ground (or place on which he 

 stands), so as to bring the hair and two black dots about level with 

 his eye. The umbrella of the principal electrode being down to begin 

 with (and so keeping metallic connection between the principal 

 electrode and the metallic case of the instrument), the observer 

 commences by taking an " earth reading." * The steel wire, with 

 a* match stuck on its point, being in position on the principal elec- 

 trode, the match is then lighted, the umbrella lifted, and the 

 micrometer screw turned so as to keep the hair in the middle 

 between the black dots. After the umbrella has been up and the 

 match lighted for 20 seconds or half a minute, a reading may be 

 taken and recorded, called an " air reading." A single such read- 

 ing constitutes a valuable observation. But a series of readings 

 taken at intervals of a quarter of a minute, or half a minute, or at 

 moments of maximum or minimum electrification during the course 

 of two or three minutes, the match burning all the time, is prefer- 

 able. In conclusion, remove the match if it is not all burned 

 away, lower the umbrella home, and take an earth reading. 



(3.) The electric potential of the air at the point of the burning 

 match is found by subtracting the earth reading from the air read- 

 ing at any instant. When the air reading is less than the earth 

 reading the air potential is negative, and is to be recorded as the 

 difference between the earth reading and the air reading with the 

 sign — prefixed. The earth reading may be generally taken as 

 the mean between the initial and final earth readings. But the 

 actual earth readings and air readings ought all to be recorded 

 carefully, and the full record kept. 



(4.) Note and record the wind at the time of each observation, 

 also the character of the weather. 



IV. Observations to be made : 



(1.) At the commencement of the Expedition, in the course 

 of the northward voyage, observations of atmospheric electricity 



* Electrostatics and Magnetism, § 375. 



