An Adantation of the Atwood Machine for Viscosity Determinations M. T. CARLISLE and CONNIE OLIVE Coker College, Hortsville, South Carolina
HE OBJECT of this paper is to show how an AtTwood machine with a falling fork may be used as a timing device for the falling sphere method of determining viscosity. A modification of Gibson and Jacob's falling sphere viscometer was used. The apparatus consists of a Ryerson acceleration outfit and an Atwood machine (Figure 1). The fork carriage has mounted on it a small buzzer with stylus, A, placed a t the same level as the recording stylus of the fork, E. At Cis a small electromagnet below which is a 15 X 1.8-cm. glass tube. The bottom of this tube is electrically connected with the buzzer, A . A small U-tube filled with mercury is attached to the fork carriage below B. This serves as a circuit breaker for the electromagnet which releases the falling sphere into tube, CD. place. A steel ball (radius 4.7 mm.) is then placed at the base of the electromagnet and tnbe, CD, filled with the test liqnid, is put in place below the magnet so that the ball is submerged in the liqnid. The fork carriage is then released. As it falls the circuit is broken a t B, releasing the ball into the liquid. The instant the ball strikes the bottom of the tube, CD, electrical contact is made with the buzzer, A , the stylus of which records the arrival of the ball a t the bottom of the tube on the record plate. The traces made by the fork and the buzzer on the record plate appear as shown in Figure 2. HI represents the point of the trace a t which electrical contact a t B (Figure 1) is broken, releasing the falling sphere into the tube, CD. This point is determined experimentally and marked on the record plate as a preliminary operation. MN represents the point on the trace where the fork stood when the ball arrived a t the bottom of the tube, CD. It is then only necessary to count the number of vibrations between H and M to find the time the sphere took to traverse the liqnid. The fork used in our determinations had a frequency of 128 vibrations per second. By estimating half vibrations, time may be read to l/%m sec. All determinations were made a t room temperature. No jacket was used for the tnbe, CD. In repeated operations a t the same temperature, using the same liquid, remarkably concordant results were obtained (Table 1). TABLE 1
In operation the fork camage is lifted until the contact wires below B dip into the mercury of the U-tube, completing the electrical circuit through the electromagnet, C. The glass record plate is then slipped into
Nrmbn of Vibrolionr
Trace 1 Trace 2 narc 3
Carbon Tclrorhlmidc
Mcthonol
Bc"$e"c
38
27
37
28
38.5
27
27 27 a7.5
To convert these data into relative viscosity a modification of Stoke's formula was used in which, 272 ( D . . d)g
.
'=
9 v
We found it necessary to multiply our results by a constant for our particular tube and sphere. Table 2 gives some typical results obtained.
(1)
TABLE 2
where
8 by Capillary
Vir~omdcr
viscosity = radius of falling sphere = density of sphere = density of liquids = constant of gravitation = velocity of fall
p =
r D d g V
Benzeneat23' Metheno1 at 23' Spiritsof turpentine 23'
h Pnllin~
Par Crnl
SYcrr
Error
0.5572 0.5550 0.9825
2.0 4.3 0.2
CONCLUSION
We confined our attention to relative viscosity. Letting p in equation (1) represent viscosity of water a t a specified temperature and = 2r"D . . . d')g
,',
0.5890 0.5310 0.9850
L
9 V'
(2)
represent the viscosity of another liquid; then, dividing (2) by (I), and letting p = 1, for water, we get as a final equation,
From our results we conclude that the Atwood machine with the attachment as described above serves as a satisfactory timing device for the falling sphere method of determining viscosity. The chief advantage in this device is that i t is entirely mechanical, eliminating the personal equation. REFERENCES
G ~ S OAND N JAMBS, Tram. C h m . Soc., 117,973 (1920). HATSCHEK, "The Viscosity of Liquids," G. Bell and Sons, Ltd.. London. 1928.
