A photocell is any of a wide range of sensors that react based on the presence of light or electromagnetic energy. They can be found in many different forms, from solar panels that use light to generate electricity, to photoresistors, which shift in resistance based on how much light is present.
Bond prices and yields are like a seesaw: when bond yields go up, prices go down, and when bond yields go down, prices go up. In other words, a move in the 10-year Treasury yield from 2.2% to 2.6% indicates negative market conditions, while a move from 2.6% to 2.2% indicates positive market performance.
Likewise, as the price of the bond increases, the running yield will decrease, and vice versa. The running yield of a bond changes from day to day as the market price changes, and from year to year as the bond approaches maturity. This is because the value of the bond converges towards the face value of the bond as time passes. Typically, the running yield of a conventional bond is higher than that of a stock.