74LS04 (5 V) outputs 1.2 V to input pins
The inputs to 74LS logic chips are emitters of NPN transistors.  If left open-circuit, they will usually be seen as high.  If you wish to input a low, you must connect the input pin to ground.

Traditionally, switch inputs to 74LS logic have used a 5K (or so) resistor from the input to Vcc, and a switch between the input pin and ground.  The 5K pull-up resistor ensures that the input will be high, unless the switch is closed.

Since the TTL inputs source current, a fairly low resistance to ground is required to input a low, so it is best to use a pull-up resistor and a switch to ground.

With CMOS parts (74HCnn or CD4nnn) the inputs are very high impedance and must not be left open-circuit.  Because CMOS inputs neither source nor sink current, switch inputs may be connected as above, or with a switch to Vcc and resistor to ground.