a a 
IV. 
A MODIFIED FORM OF ELECTRICAL CONTROL FOR 
DRIVING CLOCKS. 
By SIR HOWARD GRUBB, F.R.S., Vice-President, Royal Dublin 
Society. 
[Read, January 17; Received for Publication, Aprin 13 ; 
Published, SzpremBer 9,! 1905. ] 
Tue system of electrical control for clocks described in my paper 
read before the Institution of Mechanical Engineers, Dublin, in 
July, 1888, and adopted for the various instruments employed in 
the international photographic survey of the heavens at 
Greenwich, Capetown, Oxford, Mexico, Melbourne, Perth (W. 
Australia), &c., has been found to be capable of driving these 
large equatorial instruments with the necessary accuracy, and to 
be reliable in its action so long as the electric contacts are in 
good order. 
Within the last few years I have been able to simplify the 
system, and, by reducing the number of contacts, have eliminated 
many sources of possible error and failure. This modified form 
of electrical control has been applied to several instruments with 
success. 
The principle may be thus explained :—On the counter-spindle 
which works the driving-screw of the equatorial, and which 
generally revolves once in twenty seconds, is fixed an insulated 
disc d, d (see fig., p. 85), shod on its periphery with a discontinuous 
metallic band 8, 6. 
By a simple system of rubbing contacts (not shown in the 
design in order to avoid complication), these bands are connected 
severally to one end of the magnet-coils which actuate the 
retarding and accelerating apparatus. 
The circuits are so arranged that if “earth” (in this case any 
1 The publication of this paper has been unavoidably delayed. 
