While they vary in detail they are in principle a pipette from which the glue-solution flows at a given temperature. The time required for the glue to run out of the pipette as compared with the time consumed in the same operation by the same quantity of water gives the relative viscosity of the glue.
The viscosity test is not entirely accurate in itself, but taken in connection with the jelly test it forms a very satisfactory basis for grading.
THE JELLY TEST
The jelly test is based on the comparative resistance power of the various glue-jellies. Several mechanical devices for determining jelly-strength have been perfected. One of these consists of a brass vessel which rests upon the glue-jelly, and into which shot is poured; the weight of the cup and the contained shot upon having penetrated to a certain depth in the glue-jelly, gives a figure which expresses the comparative strength of the jelly with the standard.
APPARATUS FOR MAKING JELLY TEST
Another apparatus that has been found accurate and practical is the device illustrated on the next page. It consists of a pressure tube (A), over the mouth of which is stretched a thin rubber diaphragm (B). The tube connects to a rubber bulb (M) and to a pressure gauge, or manometer (E) with a scale (F). The pressure tube is filled with water to the point (C). The manometer tubes also contain water. A three-way stop-cock (D) connects the tubes either to the bulb (M), or to the air, depending on the position. Below the pressure tube is a brass table (G), on which is placed the glass containing the glue-jelly to be tested.
Apparatus for Testing Jelly-Strength of Glue.
When the glass is in position, the table is raised by means of a threaded wheel until the glue surface forces the water resting on the flexible diaphragm up to the fixed mark L. Then the stop-cock is turned to connect the pressure tube and gauge to the rubber bulb. By pressing the bulb the water is forced down in the pressure tube and so expands the diaphragm into the jelly, the liquid in the gauge rising simultaneously. Pressure on the bulb is continued until the water reaches the mark N.