YOU probably didn't know there was such a thing as a convection current, and to tell you the truth we didn't either until we read a report of some very interesting experiments that had been conducted recently in our Research Laboratory.

Then we thought we would see what Webster had to say about it and we found that a convection current is one which conveys or carries. In this particular case it carries chemicals which are removed from a film in a developing solution, the current itself being caused by a change in the density of the solution.

It is commonly known that when a film or plate is placed in a tank of developer vertically without agitating the solution, the top of the negative develops faster than the bottom. The same is true of the fixing of a negative. The top of the negative will fix quicker than the bottom.

On the other hand a bleaching solution works in just the opposite way. This was discovered in bleaching motion picture film. When the film is hung vertically in a tank and the solution is not agitated, the lower portion of the film bleaches faster than the upper portion.

By theoretical reasoning it was concluded that in one case there must be an upward convection current next to the surface of the film and in the other a downward current, the reason for these currents being the change in the density of the solution near the film or plate.

In the bleaching process the halogen, that is, the iodine, bromine or chlorine in the bleacher unites with the silver in the film. This halogen is lost by the solution so that portion of the solution next to the film has less density and begins to rise, forming a current.

Naturally then the freshest solution reaches the lowest portion of the film and as the current follows along the film, the solution becomes more and more exhausted.

With developing, fixing or reducing, the result would be the opposite. The density of the solution is increased by the silver removed from the film so that a downward current is produced which causes developing, fixing or reducing to be accelerated at the top of the plate or film and relatively retarded at the bottom.

While all of these things were indicated theoretically, there was no positive proof that they did occur, so one of the Kodak Laboratory workers attempted to demonstrate the actual presence of these currents so that they might be seen and studied.

After a number of unsuccessful attempts it was found that by pulling cotton apart and cutting short fibers into the solution, these fibers were readily seen when brightly illuminated. They showed the position, the direction and the duration of the currents as theoretically indicated and proved the conclusions to be correct.

While these facts are of greater importance to the scientist than to the practical worker it is well to know why it is recommended that the developer be slightly agitated when films or plates are placed in tanks for development.

It only requires a slight agitation of the developer to completely counteract the effect of these so-called convection currents and insure the film or plate being developed evenly over its entire surface.

The developer is somewhat agitated when films or plates are placed in it, but it is advisable to move the hangers once or twice during development to be sure that the image is developed evenly from top to bottom.

Short exposures need not be under-exposures. Super Speed Film is all that the name implies - has greater speed than any other film or plate you have ever used.

A Bit Of Princeton, From A Portrait Film Negative By W. N. Jennings Philadelphia, Pa.

A Bit Of Princeton, From A Portrait Film Negative By W. N. Jennings Philadelphia, Pa.

Copyrighted, W. N. Jennings.

Commercial Subject, From A Portrait Film Negative By W. N. Jennings Philadelphia, Pa.

Commercial Subject, From A Portrait Film Negative By W. N. Jennings Philadelphia, Pa.