I live in an older house. I am replacing a single pole light switch with a smart switch. The current switch has 2 black wires going into it (one into the top screw and 1 into the bottom screw). It also has 1 wire black wire going out from the bottom screw that connects to other black wires coming from the box. All of the white wires in the box are connected together. There are no ground wires. Which wire would be the load wire? Thanks.

So I have 4 troffers with 4' bulbs in each. They all worked for several years. I turned the light switch on and one troffer came on with all 4 bulbs and the other 3 nothing. Ok so I went out and bout new bulbs. Flipped the switch, nothing. Later that day, flipped the switch and here they are. Flipped it again later in the day, nothing. I finally noticed that the bulbs in the other 3 troffers were dimly light. I opened the lens and touch a bulb and they all came on, but only on the light. The other two were still off. I went back to the switch and flipped it off, waited for a few seconds and flipped it back on and nothing. Lights were still dimly light. I took a tester I have and placed it by the lamp holders and it light up and beeped. So there seems to be power there. If I move the lamp again and it lights. These are T8 bulbs and there is no noise from the ballasts. How could three fixtures do this at the exact same time? Any help appreciated in figuring this out. Thanks