The following information has been gleamed from studying a Simplex tank purchased for £1 from a local flea market. In 1937 it cost £1.17s.6d (£1.88p). No instructions accompanied the tank so there remains some uncertainty exactly how the tank was meant to be used, but there is little doubt that the essentials are as stated below.

More recently (November 2009) I obtained a Superplex tank from the same manufacturer which did have its original instructions. These can be downloaded as a pdf here. Also, study the construction of the Simplex tank and the Superplex tank together, as they are of similar construction and operation.

February 2013 update. At a Camera Fair I saw a Simplex tank for sale with a diagrammatic guide to its use. The vendor allowed me to photograph this 'guide' and this is shown below. Presumably the diagram was originally accompanied by some text, but that was missing.

The Simplex daylight loading developing tank was made in Austria by Friedrich Fisher, Vienna, during the 1930s. It was sized specifically for developing 120 or 620 size roll films.

In many respects it resembles a conventional darkroom loading tank, being constructed of black Bakelite with a push-on light tight lid covering a circular chamber containing a conventional plastic spiral (see below, left). A top central funnel allows pouring in developer & fixer and a peripheral spout is provided for solution emptying.

But there are two additional features which distinguish the Simplex from more conventional darkroom loading tanks. The most obvious is the side chamber with a thumb wheel secured light tight lid, and the second is that two thumb wheels on the side of the main chamber screw into a vertical channel and push against a red rubber sealing strip.

The spiral is conventional apart from the end plates being of fixed separation, specifically for 120 / 620 film. The two halves are held together by the screw-in funnel.

The spiral loads from the outside inwards and has no special means to assist film loading into the spiral.

Taking the top off the side chamber (a single knurled thumb screw) reveals a chamber into which the film is located complete with its backing paper (see below left). The chamber contains a 'pull out' Bakelite 'tube' with a side slot and a metal spring clip that can be placed around the film (see below).

The sealing tape around the exposed film is torn away and the backing paper unrolled a few inches. The metal clip is then placed around the film, preventing the backing paper from unwinding further (see right). It is important to attach the clip over the film in the way shown with respect to the backing paper.

The film, with the clip, is now placed inside the Bakelite 'tube' (see below) and the whole then inserted into the side chamber of the tank (see left). The backing paper is turned back on itself and is led out of the side chamber as shown. The top screw on plate is now put back on.

In order to understand how the light sensitive film, wrapped inside the backing paper, finds its way onto the spiral, its necessary to understand that the main chamber consists of an external open-base cylinder inside which is a a closed-base chamber into which fits the film spiral. The internal chamber can be rotated relative to the external cylinder (by about 70°) using the large knurled grip at the base of the main chamber.

When the internal chamber is rotated fully clock-wise, a vertical slot in its side wall lines up with the vertical rubber strip (see far left) inside the side channel that was referred to above. When the internal chamber is rotated fully counter-clockwise. the same vertical slot engages with two spring steel strips which connect into the side chamber film compartment (see left).

The picture to the left shows what happens during the film loading sequence. By not having the spiral inside the tank and without the lid, it is possible to see the film entering the tank via the spring steel strips.

In practice, with the spiral inside the tank, the tank lid on, and the spiral rotated fully counter-clockwise, the backing paper is pulled slowly but firmly away from the tank.

Inside the side chamber, the light sensitive film is eventaully uncovered but this happens at the end where the film is not attached to the backing paper. Hence, as the film comes out from between the turns of backing paper, instead of following the route taken by the backing paper, it's natural stiffness and curl are sufficient to enable the celluloid film to follow the 'tongue' on the steel clip which surrounds the film roll (see above) and to pass through the spring steel strips which are engaged with the spiral. Further pulling on the backing paper then 'feeds' the film into the spiral.

Carefully pull the film backing paper slowly and evenly always in horizontal direction, through the film chamber opening slot, as in the picture above and as shown as Fig. 8 in the diagrammatic at the top of this page. Continue until No. 1. on the backing paper appears, when special care should be taken until the film unwinds rather less easily. This resistance marks the point where the celluloid film is attached onto its backing paper. At this point, the original instructions for the Superplex tank have been consulted to obtain the following advice:

With many makes of films the celluloid film is only lightly attached to the backing paper and, therefore, detaches itself easily. But when the gummed strip is made of stronger material such as calico, etc., the backing paper has to be pulled out most carefully until the beginning of the celluloid film and its attaching gummed strip appear in the slot through which the backing paper is being pulled. Then, with a pair of scissors, cut the gummed strip so that the celluloid film is completely separated from the backing paper (see Fig. 9 in the diagrammatic above). The backing paper can then be entirely removed by further pulling from the film chamber slot. A small slotted 'forked end' brass strip (supplied in a separate envelope) is then taken in the hand and is used to push the celluoid film end back into the tank (see Fig. 10 in the diagrammatic above).
Note: This brass strip did accompany the Simplex tank that I purchased a few years ago (maybe 2006), but at the time its use was unclear and I did not photograph it. It was about 5cm long and 1cm wide, of thin gauge metal and notched at its working end. I no longer own the Simplex so cannot, now, retrieve the strip.