Most people look at this watch and think 'Men in Black', but the Hamilton Pacer's true claim to fame is that it carries the first electric wrist watch movement offered to consumers. There are two versions of the movement- the 500, released in 1957, and the 505, a later upgrade. This is the original, and it is a marvel of engineering that was ten years in the making.
There were two problems that Hamilton had to be overcome: the first was coming up with an oscillator system using electrical current that would work in a wristwatch, and the second was coming up with a battery small enough to fit in a watch case that could meet the device's demands. Engineers at Hamilton began work on the problem in 1947, and what they came up with is fascinating. There's no mainspring – the power to move the gears and hands comes from the movement of the large balance wheel, which indexes the gears as it swings. The Model 500 is a "moving coil" electric watch – if you look at the movement, one side of the balance looks more or less normal, with balance screws; the other side has a large wire coil mounted on it. The coil is an electromagnet. Below the balance, set into the plate, are two disk-shaped permanent magnets. As the balance swings, the coil enters the gap between the two magnets, and one of the two very thin wire springs you can see passing below the balance feeds a short trickle of current to the coil, via a contact on the hub of the balance.The induced magnetic field in the coil interacts with the fields of the permanent magnets, which keeps it swinging. (The balance spring is made of non-magnetic alloy, but Hamilton's engineers still had to get creative to keep magnetic field leakage to a minimum, requiring them to develop a platinum and cobalt battery.) The second of the two long, thin springs is actually a trip spring, which is tripped by a jewel on the hub of the balance, which acts to break the electrical contact between the spring. This cuts off the magnetic field in the coil and the balance is free to swing through it's full arc.
Unfortunately, Hamilton rushed to get the 500 on the market ahead of Bulova and Elgin, and it has some problems. The contact point on the balance hub was prone to corrosion and even the most minute amount will stop the watch. Magnetic interference was also an issue. The 500 has a well deserved reputation for being temperamental, but it is a fascinating piece of history.