By the 1960s, most cameras had advanced to the point of having built-in exposure meters. Accurate exposures were now quickly and conveniently obtainable. The fist built-in meters were of the selenium type, that required no batteries, since the cell itself produced a voltage proportional to the light striking it. While a huge step forward, the selenium system had the disadvantage that it was not sensitive enough to use in low light situations.

With the advent of "available light photography", a new metering system came on the scene. This was the Cadmium Sulfide (CdS) cell, that did require a power source. CdS cells are really just variable resistors that change resistance in inverse proportion to the amount of light they receive. A small stable battery was needed to assure constant accuracy. Mercury batteries were chosen due to their long life and stability. A mercury cell has an initial voltage of 1.35V which it then holds until the last 5% of its life, when it declines rapidly. Mercury cells can last for 10 years or more in storage. This stability of current over the life of the battery made it perfect for use in applications such as camera meters.

In the 1990s many governments outlawed the sale of mercury batteries due to environmental concerns. Mercury can be very toxic, particularly in the form used in batteries, Mercuric Oxide (HgO). The reason for the ban was to prevent any further buildup of the toxic components of mercury batteries in our soil and water.

This ban has caused quite a dilemma for photographers and others with equipment calibrated for mercury batteries. Mercury cells are still available is some countries but are expensive, and of unknown quality. Other battery types such as Alkaline Cells, Zinc Air Cells and Silver Cells are available, in similar physical sizes, as replacements for Mercury Cells, each type having their own characteristics.
  • Alkaline Cells have an initial voltage of about 1.5V. This begins to decline immediately upon use and continues until the last 30% of its life, when it begins a faster decline. These cells vary in voltage wildly throughout their life. There is hardly any linear portion to their discharge curve. Good for toys, bad for cameras.
  • Zinc-Air Cells have an initial voltage of 1.4V. This declines in the first 5% of its life to 1.35V, which it then holds until the last 1%, then it drops off very rapidly. They have an excellent discharge profile and can produce more current than all the others, but only for a couple of months. The life of these cells is quite short, after the seal is removed, whether used or not. They do not work well in dry climates.
  • Silver Oxide Cells have an initial voltage of 1.6V. This declines in the first 5-10% of its life to 1.55V, which it then holds until the last 1% of its life. These cells are long lived, and maintain a very stable voltage for about 96% of their lives. They closely approximate Mercury cell discharge profiles.

The meter circuit of the Minolta SRT exposure meter is a simple series circuit, as opposed to the more sophisticated bridge circuit. Bridge meter circuits are not affected appreciably by voltage changes. Minolta as well as most other camera manufacturers chose the simpler series circuit since the Mercury Cells (available at the time) were so stable that a bridge circuit was unnecessary. A series meter circuit's accuracy depends on the cell voltage remaining as constant as possible through most of it's life. What this means is that if you use batteries with voltages other than 1.35v in your SRT, meter readings can be appreciably affected.
Owners of cameras and light meters that were originally designed to use mercury cells were facing the dilemma of having their equipment become obsolete. Collectors and users of vintage photographic equipment were hard hit. Their choices were;
  • Put their classic camera on the shelf and purchase a new camera or meter along with the required upgrade in lenses and other support items. Not an option for Collectors.
  • Buy mercury cells from overseas sources (Not Recommended), suffer the expense and receive possible low quality cells. There have been reports of imported mercury cells having a slightly higher voltage (1.4) and a totally different chemical makeup. Importing these cells will probably become illegal very soon, as it should be.
  • Use the highly touted zinc air cells, and have the expense and inconvenience of changing batteries often (as often as every 2 months). Once the seal is removed an internal destructive chemical process begins, whether used or not. The often suggested practice of re-sealing these cells between use is just plain inconvenient. The more expensive, improved air cell versions do not last that much longer.
  • Purchase one of the conversion units that reduces the voltage of a "76" silver oxide cell to 1.3 volts, and fork over about $30 plus shipping for the unit (not including the battery), for each piece of equipment owned.
  • Have their equipment re-calibrated to use new, more modern batteries. Camera shops charge about $60 and up to perform this conversion, if they will do it at all. Most want you to use the less desirable alkaline cells or sell you a zinc air cell (or preferably a newer camera!).
Since I am a collector and user of vintage cameras, all of which were designed to use mercury batteries, none of the above options seemed attractive to me, so I worked on finding a more economical and permanent solution. After much study and examination of the Minolta SRT circuitry, I worked out a method to simply and cost-effectively convert SRT cameras to use the off-the-shelf, "76" silver oxide cells. This, I feel is a permanent solution to the mercury battery dilemma. I chose not to use alkaline cells, due to the fact that their voltage is constantly changing throughout their life. The silver cells are more stable and closely approximate the mercury cell's discharge profile. The difference in diameter between the "76" and the original mercury cell is easy to solve with an inexpensive #111 "O" Ring or a plastic adapter Ring. See the page "Converting The SRT To Use Silver Oxide Cells" for more details.