2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ^2 



Sheet therlo/ an alloy having a low temperature coefficient of 

 resistance, vv^as rolled out into a strip about a meter long and as thin 

 as possible. (In the first instrument made, this strip was about 

 0.05 mm. in thickness. This was about the limit of thinness obtain- 

 able by rolling between cold rollers. For the second instrument, a 

 strip of one-half this thickness was produced by rolling between hot 

 rollers. This was done by the mechanician of the University of 

 Wisconsin Physics Department, through the kindness of Dr. C. E. 

 Mendenhall.) With a straight edge, the strip was cut to one-half 

 inch in width, and then pressed out in a specially prepared die, to 

 assume the alternately flat and zigzag shape shown in figure 3. When 

 this long strip was held together in a square frame, there were formed 

 200 small triangular tubes with walls in common, each tube one-half 

 inch in depth and about 2.5 mm. on a side. The open end of this 

 honeycomb of triangular tubes forms the absorbing area of the 

 instrument. 



The advantage gained by the large number of cells is that the outer 

 ones protect the inner ones from loss of heat, so that notwithstanding 

 the very large area of the walls of the cells compared to their open 

 ends, the central cells, losing only at front and rear, change tempera- 

 ture about as much as flat strips presenting equal areas would do for 

 the same intensity of radiation. We invoke, in other words, the 

 guard-ring principle. 



Before the long, crinkled strip was pressed into this square shape, 

 each apex was coated with thin shellac, the whole baked in an oven for 

 some hours, and this process repeated several times. Thus the whole 

 strip, when formed into its final shape, was insulated, each part from 

 every other that could come in contact with it, and a current of elec- 

 tricity could be sent through its whole length. 



On the walls of the central cluster of tubes formed by the bending 

 of the strip were fastened four thermo-electric elements, of fine copper 

 and nickel wire. The junctions were symmetrically placed 2.5, 5.0, 

 7.5 and lo.o mm. respectively along the length of the tube and insu- 

 lated from it by thin tissue paper. These wires were brought out on 

 the lower end of the tubes and connected in series. The constant 

 temperature junctions were buried in wax on the under side of the 

 glass plates /, / (fig. 2) and the outer leads were soldered to the bind- 

 ing posts a, a, (fig. i). The two ends of the therlo strip were con- 

 nected by copper wires to binding posts a' , a' (fig. i). 



^ Obtained from the Driver-Harris Wire Co. 



