On Electrostatic Screening by Gratings, Sec. 405 



Washington (Major F. P.) Lecture on the Methods and Processes 

 of the Ordnance Survey. 8vo. 1890. The Author. 



r oodward (A. S.) and C. D. Sherborn's Catalogue of British Fossil 

 Vertebrata. Supplement for 1890. [Extracted from Geol. Mag.] 

 8vo. Hertford 1891. The Authors, 



f (R.) Astronomische Mittheilungen. No. 76. 8vo. [Zurich] 

 1890. Dr. Wolf. 



April 9, 1891. 



Sir WILLIAM THOMSON, D.C.L., LL.D., President, followed by 

 the Treasurer, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 >r them. 



The following Papers were read : 



" On Electrostatic Screening by Gratings, Nets, or Per- 

 forated Sheets of Conducting Material." By Sir WILLIAM 

 THOMSON, D.C.L., P.R.S. Received April 2, 1891. 



I. Grating. 



1. Maxwell, in his "Theory of a Grating of Parallel Wires" 

 ('Electricity and Magnetism,' Arts. 203205,* and Plate XIII), 

 pves a very valuable and interesting two-dimensional investigation 

 of electrostatic screening, and a most instructive diagram of " Lines 

 of Force near a Grating," which powerfully helps to understand and 

 extend the theory, and to acquit it of an accusation wrongly made 



linst it in the last two sentences of Art. 205. It is only on the 

 supposition of the grate-bars being circular cylinders that the 

 ivestigation is less than rigorous : and that supposition nowhere 

 iters into the investigation ; it merely appears in the word " radius," 

 the first line of the last sentence but one of Art. 204, and it is con- 

 licted in lines 3 and 4, and by the rest of the sentence, and by the 

 icxt sentence. (See 6 below.) 



2. The conclusion, " a = O'lla," in the last sentence of Art. 205, 

 >ndemned as " evidently erroneous," is quite correct, and very 

 iteresting. It shows that a corrugated metal plate agreeing with 



* In formula (7) of Art. 204, delete A. ; in Art. 204, delete 2 in last line of p. 250 

 Edition 1873) ; and delete 2 in lines 6 and 16 from foot of the page. 

 VOL. XLIX. 2 K 



