NEW METHOD OF INDICATING THE DENSITY OF SMOKE AS 
INSTALLED ON THE U. S. S. CONYNGHAM. 
By Rear Apmirat R. T. Hatt, U. S. N., MempBer, 
[Read at the twenty-fourth general meeting of the Society of Naval Architects and Marine Engineers, held in 
New York, November 16 and 17, 1916.] 
This system was installed on the after stack of the U. S. S. Conyngham and 
consists of a light transmitter installed on one side of stack, a light receiver installed 
on opposite side of stack to light transmitter, and an indicating meter with suitable 
control appliances installed in boiler room. 
The indicating meter is provided with a scale of smoke densities, ranging from 
dark to clear, Plate 2. 
This system was in successful operation during the official trial of the vessel 
and was found to operate satisfactorily under all conditions of smoke. 
Scale indications at various densities of smoke were checked up by a member 
of the Trial Board with observations made visibly from deck and were found to 
be correct. 
The basic principle of operation of the system is dependent on the peculiar 
property of the metal selenium, this metal, under certain conditions of crystalline 
structure, being light sensitive in its resistance to a passage of electric current, 
viz., of low resistance at higher light intensities, and vice versa of high resistance 
to lower light intensities. 
It will be manifest, if a beam of light be projected across a stack, that its 
intensity will vary with the density of the smoke. It will be further manifest, if 
a plate of selenium of suitable structure is placed opposite to the source of pro- 
jected light, its electrical resistance will also vary with the density of smoke, and, by 
connecting the selenium plate in series with a suitable indicating meter and a source 
of electric current supply, such variations of smoke density will be made mani- 
fest by variation in current flowing through meter and, by suitably arranging the 
meter scale, such variations in electric current will be indicated as smoke densities. 
The installation is shown in elevation and diagrammatically in accordance 
with Plate 1. 
The figure 1 designates the after stack leading from boiler uptake 2; 3 desig- 
nates a cast composition box rigidly secured to one side of stack, and figure 8 
designates a cast composition box also rigidly secured to stack but diametrically 
opposite to box 3. The stack 1 is provided with apertures as shown in section; 4 
designates a cover for box 3, and to which is secured, but insulated from, the seleni- 
um plate 5 (Plate 3); 6 designates a glass pane whose function is to protect the 
selenium plate 5 from accumulation of dust, etc. Means are provided on box 3 
